The mag is reliable right out of the box. Its only job is to push the ammo up so the stripper can remove it from the mag. New mags are the best mags in this situation.
The most common complaint with new mags is they usually fight you to get the last round in them. This is the reason you keep them fully loaded. You are waiting for the spring to take a set and relax a little to easily accept that last round. The question of loading a magazine to full capacity has nothing to do with its reliability.