Our ops specs allow us to take off vfr, as long as we pick up our IFR clearance within 50 miles. It also depends on your equipment. Take off in jet showing up at 250 knots on their radar in a busy environment, and they will tell you to maintain VFR. 99 percent of the time you are fine, but its that one time that will get you violated. Also, if you are taking off from podunk, usa, and if there is a lot of traffic, they cant give you your clearance until you are in radar contact. If the ceiling is 2000 feet, and radar coverage begins at 5000 feet you are hosed.