Yes you should clean them off. A little poo residue is not too big of a deal, as long as you can still see the vent. But if it clogs over, and isn't cleaned off, it can be life threatening for the chick.
Yep, yep....exactly what the previous poster said. Out of 19 chicks, I had maybe 4 that had issues with pasty butt for a few days...and the runt of the pack that had it for about a week. I just cleaned them off with warm water every day and it resolved itself. The only danger is in letting it build up and clogging the vent.