My Mega A/C barely keeps up in temps over 90 degrees. Without the rear vents, I don't think my back seat passengers would be comfortable at all unless they redesigned the whole A/C system to make it blow cold and hard. I'd insist on getting the rear vents.
The vent system runs under all of the center console and the cupholders (manual trans) and it could be a considerable expense to add it later depending on what you have for options.
The center console has a cutout where the vents are in the back near the floor. If a truck doesn't have that cutout, you will need to replace the lower half of the center console to mount the vents. The ducting under the console is simple plastic ducting that just pushes together. Forward of the console on my mega with the manual trans, the floor mounted cupholders are molded over this duct and forward of that, I'm not sure of how it attaches to the A/C system. The main duct may not have an outlet for this, but I have a feeling it's there - just capped off.