Same here. I had a set of tires that I ran 80 front and rear, and it was a fairly rough ride. Got good mileage, though. But, as independently predicted by Doktor A and a tire guy, when the front tires are at 80 they tend to wear a little more on the inner and outer edges of the tires, because when you turn the wheels they ride along the edges. Next set of tires I went back to the door sticker (under seat sticker) at 55 front, 80 rear.
My most recent set of tires, Michelin AT/2 tires, I was down to 5/32, with considerably less than that on the edges (due to ball joints, wheel bearings, and suspension issues over time). They could have gone a few 1000 miles more, but I decided to replace them this past Saturday afternoon. 6/32 minimum for snow, 4/32 minimum for steers, and 2/32 for drives, of course. With winter coming on, 5/32 was close enough. I replaced them with the M/S2 tires.
I put the AT/2 tires on the Sprinter on 02/07/11 at 361,914 miles, and replaced them on Saturday 10/10/14 at 597,546 miles. That's 235,632 miles.
Nitrogen.