Zyngawf - I've just finished a Milky Way-type top (which is similar to a Snail Trail) and was very careful to measure and square up each unit before I put them together into blocks. It sure does help, as others have said, to spend the time as you go along!
Shayla - I, too, found myself cutting 4" squares for 3" HSTs, then trimming. However, I did cut some at 3 7/8" and didn't have to trim. Sometimes I think it depended on my mood :-). Whatever works, eh?!
Happy quilting, Everyone!
Anne P - Seattle