These are the factors I consider in deciding whether to priceline or book directly with a hotel, in order of priority:

1. How much do I care which specific hotel I get? (particularly an issue in certain PCLN zones that have wide geographies or lots of hotels at a given star level)
2. How certain are my plans? (most reservations directly with hotels are cancellable--priceline is not)
3. How many points am I giving up? (usually not that many)

Then I compare the potential savings and decide whether it's worth it. Usually the points are worth a lot less to me than (1) choosing my hotel, and (2) cancellability.