This is in an email I got from Brant...
"As far registration is concerned on this district of the Arapaho Roosevelt NF, if you have a license plate that is current, you do not need an OHV sticker. If you trailer your OHV vehicle(Car, Truck, ATV, Dirt Bike, or any other wheels with motors and you don't have a plate) then you need an OHV registration. Bottom line is you must have registration to operate a motor vehicle on NF, license plate or OHV is currently acceptable. The rules also state that the registration must be displayed, so not on your table at home or stuffed into your glove box. I get a lot of people who don't want to put OHV stickers on their machines even though they have them legally registered. Unfortunately, you must have tabs on your plates or OHV sticker on your ATV, Dirt Bike etc.
I can't speak for other districts/National Forests/Parks(national or State) but that is how my rule book reads to me. Hope this answers your question.Let me know if I created any confusion."