I was told that most will pull right before closing to see if you have entered into any debt that effectts your dti. I know of someone who stupidly went out and bought a car a week before closing, the inquires were there so they had to explain them and their closing was cancelled. They lost all of their money that they had in the deposit, apprisial, inspection, ect
Some will tell you to wait. If you are unsure of how much you can qualify for or if you can qualify then it coudl be worth it. Better yet, take your own copies of your reports pulled from here to a broker and see what they say. they may have suggestions that you can work on over the next couple months to be in a better position when it come time.
In reality, a pre-qual or pre-approval only lasts as long as your credit/job situation stays stable and the mortgage market does the same. Right now, that can be much less than 120 days....or even 30 in some cases. All they are are guidelines as to what you should, under that days circumstances, be able to get approval for. In reality, they don;t mean a whole lot in a market as volatile as this is (although s full pre-approval does give you more bargaining power)