I forgot to add that even having power to spare with my 3RZ I very rarely pull hills in 5th (and have only done so to test my power or compare with a friend's truck, and I did so knowing that I am taking hours of life away from my 5th gear).
If you need to apply more throttle for some hill or even a lot of headwind, immediately downshift to 4th. 4th gear is the strongest, runs the coolest, lasts the longest, and is the smallest downshift you can make. It's a no brainer.