I run 70psi in back pretty much all year on my 2008 F250. I use my truck for work (landscaping), so I haul loads pretty regular. After I replaced one of the tire TPMS sensors (for $100, NOT covered under warranty!!) in 2010 I think it was... the TPMS system simply stopped working and I have just ignored it for the last 3-4 years. When the TPMS was working, it allowed my tires to get down to 45psi before lighting up on the dash - at least, that is what it did the one time it warned me of a low tire before the system failed. I did notice the low tire before the TPMS lit up, though.
From what I read I guess the aftermarket TPMS systems work better than what I experienced.