As said, "it depends". For some flowering specimens, after flowering is appropriate. For instance, crabapples set their buds for next year's blooms by July so if you want a profusion of blooms a good rule of thumb is to prune before June 1. However, if your client is trying to minimize crabapple production...prune after.
For conifers, we like to trim them late fall up until May-ish. Two reasons here, the pine bark beetle is a decided problem and is active any time from May through Sept/Oct (depending on the year) and most of our conifer trimming is on pines. Also pruning in the summer means lots more sap and can mean a lot messier job.
So you basically need to know the species you are trimming and your goal, what are you trying to achieve?
Sylvia