If you are going to do a timing belt the easiest way I have found is to use a paint pen and mark either side of a tooth on the old belt and the tooth itself on the gear, count the teeth on the new belt and mark it the same as the old one.
Fit the belt as the old one came off and tention the belt.
This I find eliminates the slight movements in cam gears as the belt is removed or when you release the tention from the old belt.
It also removes any mucking around lining up timing marks etc.
works for me...