I've never had a problem - I've hiked nude to Scenic hot springs and through Baker state park. And yes, even run into hikers along the trail, who mostly laughed and called me crazy and/or brave. :-) I'm not super outdoorsy, so don't have a ton of experience, but the experiences I've had hiking nude around here have all been positive.
I agree with barebones and starhammer. You often don't have to go very far into a national forest to find places, especially on a week day.Earlier this year I found a trail that was in a designated wilderness area less than two miles into the forest. There was one other car in the parking so I initially kept shorts on. After hiking a short distance I met the people heading back to their car. We talked for a few minutes, I asked if they had seen anyone else on the trail and they said no one else was around. After they were out of sight, I became a nudist again and hiked to the end of this scenic trail. After spending a half hour by the river at the end of the trail, I hiked all the way back to the parking area nude and didn't see another person. This trail is published in several hiking guides and is close to developed areas, but during the week I was able to hike most of it nude without seeing anyone.
National Forests typically adopt local laws into their rules (county and state). Not all but many do. Therefore in Washington State it is technically illegal to hike nude in most National Forests. That said, it isn't a priority for them unless there is a complaint from the public or you happen to be caught nude at a trailhead or campground or other facility (which is actually illegal in all Forests due to a federal regulation).
The same is true for many National Parks. Sometimes it depends on when the park was created in relation to the date statehood was granted, which can get confusing. In any case, the best thing to do is just ask first. Many park and forest staff recognize nude recreation as a valid use even if it's not technically legal at their particular unit. They can often suggest places/times to hike that are free from conflict with other users.
Hope this helps!
With a good map, you need not be restricted to trails. I have found dead-end logging roads, or sometimes decommissioned ones, and hiked nude for miles without seeing another person. If the road is dead-end or decommissioned, all you need to do is see if there is already a vehicle parked; usually there isn't.