The golden rule
If it flowers before June, prune after flowering. If it flowers after June, prune in late winter. That single distinction prevents most of the damage we are called out to put right.
Spiraea in full spray is the reason we wait. Cut it in winter and you take next spring’s flowers with the old wood.
The purpose of pruning
We cut to let light in, to renew old wood, to keep a plant’s natural habit, and sometimes to take fruit or flowers. We do not cut because a calendar said so.
Tools matter
Sharp blades, clean cuts, and a wipe with methylated spirit between plants. Everything else is judgement — and judgement only comes from looking at the plant in front of you.