I have not, but I am thinking about doing so eventually. They're around 20 years old now, and I have the impression that the front is riding a bit low due to wear and loading (and, when going over bumps, that the front springs are not as stiff as the rears with the air helper springs).
I did have to get my ball joints replaced a couple years ago (in Whitehorse, YT), and the gentleman at the shop that I went to said he often recommended the installation of air helper springs for the front suspension of motorhomes on the E series chassis--and even more so for shuttle busses, which he said frequently end up with more passengers at the front than the back and so are quite rough on the front suspension. I assume stiffer springs would accomplish much the same thing. I think that the camber angle on the Ford suspension does change a little with suspension travel, which can lead to extra tire wear etc. if it is consistently heavily loaded.
