Given that you don't pay for the "reservation" for the Florence museums unless you use it when you show up, there's really no reason NOT to reserve your admission time in advance unless you just can't be pinned down to a schedule.

I was at the Accademia in late November 2007 and there was, in fact, no line at 3 in the afternoon so we just walked in and didn't pay for or use our reservations. In a different circumstance I might have been very glad to have had them.