I'm working on a type set of US coins - it's my favorite ! Always has been. There's no way I'll be able to afford ALL types, so for the 18th century stuff I settle on one representative of each design motif. That way, I don't need a specimen of EVERY Draped Bust - one good one will do me. I'm shopping for a decent 1795 half so I can check off two tough motifs - the Flowing Hair and that funky eagle on the back.
One of the problems with being overly "type focused" is it's harder to develop true expertise in any given one. One's attention is diverted over the entire range of US Coins. I think it's wise to choose a series, really dive in to the literature, look at hundreds of specimens, and develop a little specialist knowledge.