Assuming VFR conditions (otherwise it's obviously moot), the answer depends on where you're flying.
In relatively quiet airspace with a good chance of getting what you filed, I'd pick it up in the air.
In busy airspace (ie, Chicago/Socal/Norcal/Orlando/Regional/New York tracons), I'd pick it up on the ground and advise you can depart VFR. They will issue a clearance and squawk, with instructions to maintain VFR, no release time. You depart, call tracon in the air (they know you're coming), they radar ID you and then, when separation allows, either issue a hard IFR altitude, or in New York's case, they'll just spell it out, "consider yourself IFR..."