The benefit of properly trimming trees is to eliminate common problems that become more costly over time. The improvement and restoration of the look, structure and stability of all trees is a priority. Annual tree maintenance has proven to be a successful practice, which is what I recommend.
If trimming is done properly and responsibly it may be done year round. As long as the equipment is sanitized before each project and wounds on Oaks are sealed right away. Preferably, hire a professional company that adheres to the basic principles set by the ISA. Some HOA's restrict tree work at certain times of the year; I applaud these efforts as it restricts improper trimming.
As a rule of thumb, trees should never be trimmed more than 20% of the foliage at any given time. Leaving just a little foliage at the ends of branches (lion tailing) is not healthy for trees. We recommend removing dead limbs and branches, raising canopies and a general central cleaning, leaving as much foliage as possible. That’s one of the ways trees feed themselves through photosynthesis. Removing too much foliage compromise overall health.