Rover, the answer is "it depends". This has been a very good and relevant discussion on an old thread. New owners (mostly of older Mini-14s) have this question, so it is always good to have this discussion - primarily because early Mini-14s were stamped ".223 REM", as my 181-series is.
In the end (with the exception of the Target model), M193 5.56 is safe to shoot. I personally stay away from M855 as bullet weight accuracy seems to be best for all of my Minis with a 55gr bullet (similar POA/POI/ballistics). My newer Minis can nicely handle M855s, but I prefer to keep only two types of ammo: .223 REM 55gr and M-193. With the exception of my Savage Bolt gun (.223-only), all my .223/5.56 chambered carbines are good-to go should I grab a handful of ammo in the dark to load. Simplifies the logistics...And I'm a retired logistician.
A barrel marked 5.56 NATO shoots both and a barrel marked .223 "should" only be used with .223. Not sayin it can't, just sayin you shouldn't. Really no argument unless you think the barrel is incorrectly marked.
Again .... some people just can't read or comprehend. As I stated before, a barrel chambered for 223 Rem will safely shoot 5.56x45 but not 5.56 NATO. A barrel chambered for 5.56x45 will safely shoot 223 Rem but not 5.56 NATO. A barrel chambered for 5.56 NATO will safely shoot 223 Rem and 5.56x45.
Older, & I mean real old minis, maybe not, but, as another poster stated, this comes up quite a lot. I'm sure a lot of guys by older minis on Gunbroker, with no manual, & it is a legitimate question. Another poster I think supplied the dates of a .223 only mini. And if there is any question, you can call Ruger.