I think the OP's question has been answered, but I'm bored so here's my $0.02.
Lived in AK for most of my life; spent 3 years in Eastern Washington and unfortunate regular stints in Ewe-stun.
Alaska and Eastern Washington approach:
- Never checked any regs re: Chains. I always used my own judgment; was never stopped for "not having/using" chains. A couple of times I went over passes in Wa. as they were being closed behind me but made it through OK following my chain philosophy.
- Always carried a minimum of 3 sets of chains: Drive and front axle for the truck and one set for the trailer. I always used 4WD vehicles so that makes a difference
- If the weather was heavy snow so that chains were required to get enough traction to push through, I'd chain up the rear (drive) wheels;
- If the weather was such that the issue was slipping and sliding...but only if slippery enough that there was no alternative to chains (a VERY rare event), then I'd chain up both front and rear axles on the truck.
- If it was slippery enough to require chains on front/back, then I'd also chain up the "first braking" axle of whatever trailer I was towing, i.e. the axle that had the most braking effect, due to either loading or brake design.
- FWIW, I think I can recall less than 10 times in 40 years that I've put on all 3 sets of chains due to ice/snow.
- Further FWIW..and not related to the OP's question, but back in the day when my daily pickup truck was also the "get to the cabin through the bog" truck, I'd chain up all 4 wheels...makes an amazing difference in mud. Never got stuck IIRC.