If a fully loaded magazine can be easily loaded under a closed slide, then I'll top off the magazine. If the magazine must be forced in, then I don't. It is that extra degree of force necessary to insert a fully loaded magazine that tends to weaken magazine springs and cause failures to feed or chamber.
Also, police practice three forms of reloading: emergency reload, tactical reload and tactical magazine exchange. The first is with a gun shot dry and in slide lock. The second two bring the gun back to full capacity after an unknown number of rounds have been fired. And yes, don't think you can count your rounds during an actual exchange with a bad guy. So, I may also carry my spare mag one round short if it cannot be easily inserted under a closed slide when loaded to capacity.
Int'l Assoc. of Law Enforcement Firearms Instructors