I don't keep anything that isn't a wonder (Mystic) or basically 80%. But I also don't keep more than three of anything unless it is in the 90s.
I do bend the rules for legendaries - I have transferred a couple but generally keep them all.
Also, I have some in the 80s that I already put a lot of stardust in (my highest Vaporeon is only an 84) and I keep those as well.