In my experiences I have found that it doesn't make any difference in most occasions. Some people like to think that a hotel might have allocated 50 rooms to priceline and they break them down like this:

2 rooms at $30
4 rooms at $32
4 rooms at $37 and so on .....

so that if you are first one in the door, you could bid for 3 rooms, and not get them unless you bid at $32 where you could save money bidding 2 rooms at $30 then 1 room at $32 (I know I have made this really simple and we are probably talking about bigger intervals.

What I have found is that the hotel with 50 priceline rooms might be something like this:

25 rooms at $30 and
25 rooms at $50

so if you needed 2 rooms, it would only make a difference if the first room you bought happened to be #25 or #50.

This has been my experience, I travel with a group who bids on a block of rooms, then tries to go back and add rooms and it makes me believe the second example is true, until someone proves it otherwise.