I've always been told to install a top in average temperature, not hot and not cold. This way, when it comes time to put the top up, it will fit correctly; if you install on a hot day, you'll need the same hot day to be able to stretch it enough to get the snaps on.
Of course, if you install on a too cool day, it will sag like Aunt Nellie's Knickers on a hot day.
Something to consider.