The idea is that you want to get as much air out of the master cylinder as possible before it is hooked up to the rest of the brake system. That does a few things, one, it ensures that you don't push any more air into the system than is already present. Two, it also ensures that you get all of the air out the ports in the master cylinder itself, which can be very tricky to do. This also has the side benefit of preventing cavitation of the fluid when bleeding the rest of the system, especially if you're bleeding it manually.
That said, not all master cylinders require bench bleeding, a good example would the single circuit master cylinders found on pre-'59 trucks.
You don't mention in your post what type master cylinder you are using (stock? dual circuit?) but if it is one that should be bench bled, you should be able to do it in the frame. Bleed it in the same manner as on the bench, just do it before you hook the rest of the lines up.
A little tip if you should ever have to replace one where the rest of the system hasn't been touched to make bleeding easier/quicker- After bench bleeding and installing the master cylinder, install the lines leaving the fittings slightly loose. Then have a helper slowly push down the pedal while holding a rag around the fittings, then tighten them up while the helper holds the pedal down. This will push out any air that's in the upper parts of the lines, instead of pushing it further down. You will still want to go ahead and bleed the system to be 100% sure there's no air, and also to flush out any contaminants in the system.
BTW, I'm in agreement with Hollow65, pressure bleeding is the best way to flush the system out and bleed it at the same time.
