I've played bass and taught indoor bass. Five basses really lend themselves to exercises in that you get four eighth-notes of beats then either a resolve on the bottom or start reverse the pattern.
Four is too few and six is unnecessary. If you simply double up the big notes on six, you'll create an opportunity for errors with sloppy unison notes. More than six (except as a brief gimmick) and you make the problem worse.
If I was on staff I would keep more than five for the purpose of keeping good members in the organization when there aren't obvious cuts.
