Echo what others said...book the best refundable rate you can find the moment you think you're going to take the trip. (Or, the moment you've bought the nonrefundable airfare.)

For me, Priceline is either for one of two cases:

- 5* hotels in their nomenclature (I realize these are often really more like 4* hotels), especially when I am likely to get a Fairmont, OMNI, or other niche brand that doesn't penalize a lack of elite status or penalize a Priceline user for, well, using Priceline. Often booked the morning of the stay, or now (with many "regular" rates having a 1-day cancel rule), 24 hours prior.

- Rural 2.5* hotels where elite status is irrelevant. Cases where my choice is to pay $90 for an HIX or $40 for the same HIX and not earn points. I will often book these *after* I drive around the hotel and confirm that it's a clean-looking place.