A word of warning on all P.R. valves
This is not a chop to the quality of these valves, because most are made quite well.
What everyone needs to understand is that almost every raft any repair shop takes in with blown floor I-beams had a pressure relief valve. If they really worked, as opposed to providing a false sense of security, these shops would see far less of this problem. As it is, too much reliance on p.r.'s provide steady income for repairmen.
As was already stated, they tend to vent too slowly to keep up with rapid pressure increases on a hot day, and while sand can make them leak, much worse is when sand prevents them from opening.
So yes, clean or replace by all means, but better still use a pressure gauge if you don't know what the appropriate amount should feel like in your floor.
For boats with welded I-beams and/or perimeter seams, 2 p.s.i. should be okay. For hypalon boats, it depends. Hyside and NRS seem to have exceptionally solid glue work, as would the vulcanized Demaree's, and 1.8 to 2 p.s.i. should also be fine with them. With older rubber boats (i.e. Riken, Momentum, etc), or Avons or Achilles, I would not go above 1.5 p.s.i.