It doesn't matter at all as far as succesful transplanting is concerned - rhododendrons and azaleas are extremely easy to move and they are routinely moved while in flower for garden shows and other displays. Waiting until after bloom makes sense in that you'll be able to enjoy the flowering show and you'll be placing them in a new location at the time when they about to start growing most vigorously.

I hate contradicting mainegrower - if I am :), but I would say you can usually move them successfully any time the ground is not frozen or they are not in active growth....the period of active growth being that time immediately following flowering. You run the risk of those newly opening foliage buds wilting, not recovering fully and can lose a seasons growth if moving just as those are elongating, unfurling. Better could be to move before, or during bloom, but not immediately after bloom unless waiting long enough the new leaves have had time to be fully formed and somewhat more mature.