Some of these things are just myths that date back to GI 1911s. Remembering back on my initial training on the 1911, the instructor said "never drop the slide on an empty chamber". When asked why, he stated "because it will batter the frame and slide". The instructor also said "always load from the magazine, never do a single load and drop the slide". Again when asked why, the instructor said "because you can get an accidental discharge". I took this as gospel for many years until I started working on a lot of 1911s. I found little merit in the "slide and frame battering issue", however when the slide strips a round from the magazine, it does slow it down a lot. I have seen quite a few broken barrel links, peened slide/barrel lugs, and even a few broken or bent slide stops but it impossible to say if it came from normal firing or from dropping the slide on an empty chamber. To this day, I still ease the slide down on an empty chamber .... mostly because I had this concept drilled in my head.
As for single loading .... there is indeed some truth to this issue. With 1911s (Series 70s and clones) and many other pistols, the firing pin is an inertia device and will thrust forward when the slide drops. I have actually seen a few 1911s fire when "single loaded" (not from a magazine). I have also seen light dents in primers from charging a 1911 from a magazine, especially in 1911s where the recoil spring has been replaced with an increased power spring. Most increased power 1911 spring kits also include a stronger firing pin spring that reduces firing pin inertia thrust.
All Ruger P-series pistols and many other brands have a firing pin interrupt device. This prevents the firing pin from traveling forward unless the trigger is pulled. In fact, the P-series Rugers have linkage where the firing pin remains locked in position until the slide is in full battery AND the trigger is pulled ... very safe. Series 80 Colts and some clones also have a firing pin interrupt device. The firing pin interrupt device makes it safe to single load without stripping from a magazine but could result in damage to the extractor.
When a pistol cycles normally, the fresh round from the magazine is stripped and the head of the case slides up against the bolt face and into the slide's "pocket". This places the case rim under the extractor. When you single load without a magazine, the extractor must cam off the case rim while under considerable spring tension. If the extractor is shaped properly (some are, some aren't), it won't hurt a thing but if the extractor is misshaped, it will bend or break the extractor. So basically, if your extractor breaks or bends, it wasn't right to begin with and would probably fail anyway.
Charging a pistol by releasing the slide stop vs the slingshot method ... lots of chatter concerning this issue. First, it's less likely to cause damage to the slide (or bolt) and the slide stop (or bolt stop) if you use the slingshot method. That said, I was trained to release the slide by pressing down the slide stop and always charge a pistol in this manner (assuming it is equipped with a slide or bolt stop). In my opinion, it's much faster to do a rapid reload when using the slide stop instead of taking extra time and both hands to slingshot the slide. My old Series 70 1911 has 10s of thousands of rounds fired and still locks back perfect after the last round is fired. All Ruger P-guns use the same basic 1911 design for a slide stop so I doubt if you will see much wear by pushing down the slide stop vs using the sling shot method. I think it really boils down to how you were trained.
One exception is MK II and MK III Ruger pistols. The bolts in these pistols are quite soft so the lock notch tends to peen. Most of the peening comes from normal use; however, charging by pressing down on the bolt lock does accelerate wear. I found with my old MK II, after many years of use and 10's of thousands of rounds fired, the bolt would fail to lock back after the last round was fired. The fix was very simple ... just take a file and dress the bolt lock notch where it is square. Yes, this does allow the bolt to move a bit farther forward ... but only about .020~.030". I did this a couple times over the the years and found it had no ill affect on the pistol. 22/45 pistols don't have a spring loaded bolt lock so you have to hold the bolt lock down while you slingshot or just press it down to charge the pistol.
Just another quick note .... no amount of oil or grease will prevent peening. Because MK II & MK IIIs have "internal" bolt stops, they tend to accumulate powder residue. Oil or grease may make the bolt stop work easier but in time, abrasive crud will accumulate on the lubrication and will accelerate wear. P-series Rugers and all other pistols with slides have "external" slide stops so they don't tend to accumulate crud and can be lubricated if you so desire. I run my pistols dry ... except for a very light oil film to prevent rust.
Edited to add:
easyrider604, Again, some myths involved. Sometimes a 1911 hammer will drop to the half cock or safety position when the slide is allowed to drop on an empty chamber. There are three reasons ... either the sear has been altered to the point of being unsafe or the left tyne of the 3-finger spring doesn't have enough tension. Both are dangerous conditions ... (think going full auto). Another thing that will cause the hammer to drop is the weight of the trigger. When the slide slams shut, the trigger will actually bounce and release the sear. This sounds bad but in actual operation, your trigger finger will dampen trigger bounce. Most extended 1911 triggers are made of aluminium or are drilled out to reduce mass and prevent trigger bounce.
Charging a 1911 with the trigger pulled can be a very dangerous procedure. Assuming the disconnector is working perfect, the gun should not fire .... however many old 1911 guys have dumped a round while following this procedure. I personally witnessed a guy that had been doing this procedure for years without a problem ... then one day it happened and the disconnector failed leaving a nice hole in the shooting bench. The look on his face was definitely a Kodak moment.