Obviously drivers have to do a lap to the grid so they are always burning off that much fuel to get around, but if a driver wants to they can do more laps before the start of the race (normally if it has rained the night before or something and they want to get a better feel of the track at the expense of their fuel) but they have to then come back into the pit lane to go back out for another lap so they don't cause any disruption to all of the people on the grid. Once they go down the pit straight they have to stay in their grid box till the formation lap. Of course they are restricted by time, they have to be on the grid in enough time for the start of the race.
It's not very commonly done, expecially by those in the top 10 who have to qualify on their fuel loads... why would you carry extra fuel in qualifying and then not even use it to go longer in the first stint? If you're going to have a shorter first stint you may as well have had a higher grid slot...
Hope this makes sense...