We are doing exactly that- picking the flowers and putting them in the freezer in order to build up sufficient quantity to make the wine. We only pick those that are fully open and dry, those that are not quite fully open will be picked the following day. Collected 3 pints of petals so far.
I know the old saying is to pick on St.George's Day but we missed that last year as well and still produced a pretty decent wine for our first attempt.
I did Google about freezing them and found others had done so.
I'd say go for it!