You know the old saying "A bird in the hand is worth two in the bush?" It's especially true when dealing with RPs.
They raise the points needed to cashout without warning. They fold and disappear into cyberspace. They suddenly go from sending rewards in a timely fashion to taking months. Even MyPoints could take a powder, leaving you high and dry.
Since the highest webbie that MyPoints offers is the $25 webbie for 3500 points, it makes no sense to sit on any points above that amount. Cash in as soon as you can.