I put 3 on attune for all my pirates, and it seems to be enough in most situations. On the current pirate I'm working on, I don't plan to put anymore than 3 on attune.
For me, and my playing style, the ones that are a must to max are poke, heal, and life drain. With those 3, plus the 3 on attune I already used 18 of the 25 skills
