If you have to, best time to risk it is early June - they don't really appreciate being pruned, though its okay to snip off small bits here and there.

I have always pruned camellias with never a problem, large specimens taking off a third here and there over a few years so it’s not too obvious and this keeps it flowering well, even renovation pruning where you take the whole camellia down to a few inches from the ground and always comes back with vigour, this I always do once flowering is over, April/ early May.