My understanding is its ok on forest service roads if it's street legal.
Soooooooooo just a tiny off topic
If you are street legal. Than good to go on National forest roads? 
No. If you decide to ride in any NF in Washington, call and talk to the RANGER who runs that particular NF and ask. Each NF is ran by their own RANGER who decides if they will let ATV/ORV's ride in their own NF. Each one is different, so be warned you can get tickets big time.
If you are allowed to ride, make 100% positive you have your Drivers License, registration, current tabs, goggles and a big one, your HELMET and wear it, like it or not. That's the one that seems to get ticketed a lot. WDFW is big on not wearing helmets. I think it's a $124 for sure ticket.