Hello,
I collect rare trees and plants in general, have been since 1988. As far as your trees are concerned, just let them grow and trim as needed. Once you start hacking away at a tree it will only make it grow faster (and weaker) Time to prune?, it don't matter, but in winter you can see the trees structure better. try to remove branches that are crossed. The birch I would just let grow without any pruning, I don't know about where you are but the birches here in NJ are having major problems with disease. Never cut the leader out of a tree, it will only make it short and fat. Never cover the cut with anything, it will only harbour disease and keep the tree from healing naturally. If you need to cut larger branches, find the collar of the branch (thats the radius part where the branch meets the tree), cut up from below, then finish cutting the branch off from the top, this will keep the bark from tearing. Try Felco products for maintaing trees, they are the best (but expensive), the nice thing is you can buy replacement blades. I hope this helps.
Mike.ar
BTW, I do bonsai as well.