One exception is with plants that don't go dormant in winter, in which case frost can get into the cuts. So many evergreens are usually reckoned best pruned a few weeks before frosts. Sept is a standard time for pruning evergreen hedges.

Another exception is with some flowering plants that form their flower buds the previous summer, and you'd lose the flowers with winter pruning. Forsythia, Philadelphus and Viburnum tinus, for example.

Summer pruning is a common technique for certain fruit crops.

But against these rules I prune my beech hedge in late summer - I want a tidy hedge, and so I get rid of all the straggly growth as soon as it stops its major growth spurt, but in time for it to open a few leaves to cover over the damage.