I wouldn't think it would give you any measurabe difference in mileage as its at the tail end of the exhaust system and the exhaust has already gone through the ex manifold/header, cat and muffler before getting to the part you want to cut off. Another thing to remember is that section of the tail pipe is there to let the exhaust flow away from the truck. Without it, the exhaust fumes could gather under the truck, say as you sit in a traffic jam and cause those fumes to go up into the cab. Unlikely, I admit, but I would say keep the tail pipe. I doubt the fumes would be enough to be fatal, but I've rode in vehicles with exhaust sneaking its way into the cab and it sucks when you get a headache whenever you go somewhere.
This answer right here hits the nail right on the head! I used to have a muffler with a turn down installed right above the cab, its louder but exhuast fumes crept their way into the cab, major safety issue, not only bad for your health to be breathuing those fumes in but stinks up your interior as well. I just got a complete new exhaust system installed about a month ago, all LCE and painted flat black, it's hidden very well, very low key and the problem is fixed, I love it, here's a pic of where it exits out. It's not as loud but I'm not driving a race truck, my truck is slowly becoming an elegent Mini Truck.