There's no need to prune for the sake of flowering - they'll come back and flower really well next year even if you leave them alone. So you're pruning just to keep it within the space you have for it.

My approach is to leave alone as much as possible, cut off any bit that is really encroaching, definitely leave any strong shoots from the bottom, and occasionally take out a big old stem from somewhere near the base. I've no idea if this is the recommended approach.