Doesn't matter if it's dig or roll. The only reason I would prefer roll is it seems to be less of a shock to the drivetrain letting me save the dig racing for the actual track. It's quicker to pull off and then 'get back under the radar', so to speak.
Second, many races seem to take place in an area that can cause serious traffic problems if you try to go from a dig. I always hear how dangerous high speed racing is on the highway, yet people will stop on the highway to race and block traffic without realizing how much more dangerous that it. When you're drving on the highway, you're not expecting the automobiles to be stopped or near stopped in the lanes. By the time you realize it, it could be far too late and you're rear ending a stopped car. I know, I've seen this happen while runs were being setup at a place in Texas.
When I do go from a run, of course I prefer to be in a sweet spot for my powerband. Often, that's a bit higher speed for the gear in my car than most, though. Many people with a stick, 40 is a good 2nd gear roll for them whereas, 50mph is probably alot closer to what's good for me. More than anything, my surroundings control what I race from/to and when I race more than anything. I have no problem refusing to race people on the street if I'm not comfortable with traffic, etc. I like to save it until I"m out in the middle of nowhere, mostly.