In my '03 5.3 Tahoe I rarely floor. I keep the throttle steady and at 50ish I downshift to second and keep it about 3800 rpms. This is in the Rocky mountians and it seems to be very happy pulling at the high 3k rpm to 4k. I also hold it in second until I can let way off the gas and let it shift easily into third. My TH is about 6500 normally loaded and my GCVW is in the 12,500# range. I usually get just under 10 mpgs pulling in the mountains.
It seems like at 3400 rpms and less it starts to really struggle. It's always better for the engine to pull under load at higher rpms.