Calling up on the radio and getting a pop-up from your present position might not always be an option...
You need to be in radar contact, and at or above the MEA/MOCA for your area in order for the controller to just activate your clearance with vectors on the fly.
If you're going to rely on a pop-up as an escape route you need to know the MEA's, airways, and fixes along your route. Worst case, you need to be prepared to give ATC (or EFAS) a starting fix, route, requested alt, and destination.
Also be advised that they will only accept pop-ups if their workload permits...this could force you to declare an emergency.
If you are flying in mountains, be very careful with this...that canyon you're scud running through is probably below radar and the MEA
The safest thing to do would be to file VFR to IFR well in advance...that way you select the fix at which you go IFR, and ATC will be expecting you.