i am second guessing myself also. my only experience with this has been with that pair of 6x9's that literally fell apart after a few days of hard use--the voice coil sounded like a slinky . oh, and they only had a aftermarket deck(50x4max, 22x4 rms) powering them, so there was not too much power going to them.
i have never had a problem, or heard of problems with voice coils falling apart when they are installed using correct polarity.. but then again, i haven't heard/read anything about installing them using reversed polarity...
my other thought is the difference in voice coil construction between subs and mids. it could be the glue, wire, or maybe the former?
maybe some more experienced audio experts can chime in?