I always have my books pressed prior to submission. Not doing so leaves cash on the table, and if I am going through the process I want to maximize what I get out of it.
The trickier part for me is that I use a third party to press and sometimes clean, and they send the book in to CGC for me. That means paying for shipping to the third party, paying for the clean and press, paying for shipping to and from CGC, paying for slabbing, and then paying for the finished slabs to come back to me from the third party. This means planning an order of about 16 books to minimize the shipping fees per book.
I've found over time as the pressing fees have gone up that I am getting priced out of this type of process. I bought my own press and now it is a matter of practicing and getting good at it before doing a submission as a trial run. I may leave a bit on the table because I wont be as good as the guy I was using for pressing, but I still may come out ahead at the end of it from the amount I save by doing it myself.