They will clearly put it in their manual. Some of the military surplus 1911 mags are designed not to seat with a full mag on the slide forward. IOW capacity can be no more than seven. I have one of those mags, the mag is always used for a reload as opposed to carry.

Glock frame rails break under extreme stress, the cycling does not stress them, but a mag rammed home that has NO play does. On new Glock mags it is darn near impossible to load the last round without a loader.

It all boils down to personal choice. If I needed that one more round if the gun already holds 10 or more rounds, then I will probably be a victim if I can't stop the threat with less than 10 rounds. If the rails break on my Glock when I need it I will end up like the officer that lost his life when he needed his.