Yes, maples (especially) will bleed profusely in the spring, but if the tree is healthy, it won't hurt it. I wouldn't worry about it if you are just cutting a few 1.5 inch or smaller diameter branches, but if you need a bulk trimming then it is much better to wait until mid to late summer rather than early fall. This way, the the tree wounds will heal faster. early fall is certainly acceptable, though.
If you cut in spring, if the sap flows down on the tree bark, it's likely that the tree bark will be permanently stained for a few years.
