I've done it twice, no three times now. I usually start bidding about 30 days prior to my trip.
The first time we had to have a minivan. I started bidding at $28/day, then $30. About 24 days before leaving, they took my $30 bid. I have a feeling that acceptable rates change 30 days out, but I don't know that for certain. That's one reason I wait.
The other reason is that they charge you credit card when you get the reservation. Since Priceline has been considered financially iffy at times, I didn't want to bid any sooner than that.
I've gotten National, Budget and Alamo and never had a problem. Except for the price of the refill gas at Alamo. Stop at the station!