Each PC hotel has to have 5% of their total rooms open to PC rewards availability. There is really no reason not to go higher though, unless you are going to sell out. Even then, the incentive is small.
The guarantee for a room is 72 hours in advance and points can't necessarily be used. They have X amount of special event days they can use per year as well. Some hotels can be dicks and jack up their rates if someone tries the 72 hour guarantee. If you wanted to use points/guarantee, I would book the room normally then call the hotel and try to get them to ok using points. If they know what they are doing, they won't, but most HIs are run by dumbasses