do it now should be ok for flowering in the summer.
But yes on some species of buddleia they say prune after blooming depends on which one you have got.

This particular Buddleia is a variety of B. weyeriana - recommended pruning for that one is as for Buddleia davidii, which is prune hard late March or early April. Some of the various websites selling it, though, recommend pruning hard after flowering, very odd, because that would mean pruning hard in autumn, just before winter sets in, so I don't know what they're on about really. I'd stick to the recommended pruning for B. weyeriana, since its a sport of that anyway, but it is said to be 'compact' which might mean it doesn't need quite such drastic pruning as B. davidii, which can grow 9 or 10 feet in a season. So I'm saying prune now, depending on its size as to how much you take off.