The "Rule of Thumb" for pruning blooming plants is to prune after they bloom. Azalea (Rhododendron), Flowering Crabapple, Forsythia, Hawthorn, Lilac (Syringa vulgaris), Magnolia, Mockorange (Philadelphus), Mountain Laurel and Rhododendron all bloom in the spring and it is imperative that these plants be pruned/cut back now so that they can grow all summer with out getting too big. If you do not cut them back now, they will continue to grow during the summer up to a point where they will simply be too large and by late summer, cutting them back would mean that you are cutting off next years blooms.
We have 8 magnolia shrubs here on the property that are drastically pruned each spring (after blooming) and it is unbelievable at just how large they get by the end of summer. We can't wait until the blooming is over the following spring so that we can get them back down to size. It is not enough to lightly trim back the plants where the blooms were but you must get crazy and cut them way back knowing that they will grow to a good size again during the summer. It is not too late to do the necessary pruning of all of the plants listed in the first paragraph so get out there and start pruning.
There are several blooming annual trees and shrubs that benefit from light pruning such as your hibiscus, gardenias, lantana, roses, salvia, snap dragons, and many others. But the reasons for pruning these plants has nothing to do with the results for next year, but rather for what will happen next week. Many of these plants, after blooming, go into making and producing seeds and seed pods, all of which tend to slow down the production of more blooms. And after all, isn't it more blooms that we are after? Why not occasionally do some light pruning, for the sake of more blooms. Leave seed production for the professionals.