While it is possible to trim or shape a plant any time of the year, timing is very important when it comes to pruning.  As a general rule, if the shrub or tree blooms after June, late
Winter is the best time to prune.  Because these plants, such as grapes, bloom on current season's growth, pruning will stimulate good branching and healthy Spring shoots.  Now if the plant blooms in the Spring, then you will need to prune after it flowers.  The reason is that these plants bloom on last year's growth and you would probably remove the flower buds if you prune too early, good example would be an Azalea.  The least desirable time is right after new growth develops in the Spring. 
 
If now is the right time to prune your plants, then don't just give them a haircut.  Too often we see plants being pruned by just lopping off the top few inches of the shrub.  When this is done the shrubs turned into something that resembles meatballs.  Perfectly sheared spheres and cubes make a mockery of the natural form and beauty of the plant.  Take your time and do it right by selectively pruning branches.