I think it would be useful to prevent negative user points. I changed my points so users start at 20 instead of 100, but this means two downvotes and they're in negative points.
If a user gets a new downvotes early on, they may get discouraged by a negative score. It would be better IMO if all users started at 0 but didn't go below that.
The only problem I see is that if they get two downvotes, then one upvote, a recalculation would send them back to 0.