Only time I've ever been outright dishonest while hitchhiking was when I needed to cross the UK really fast, and thinking back to the charity fundraiser hitches I did in college, I made a sign saying "Hitchhiking for Charity". I didn't have too much trouble, I only had to lie to every driver who picked me up, which I did a good job of since I'd done fundraisers in the past for Cancer Research UK.
A lot of drivers gave me cash or change for my "cause"...god as my witness, I dropped every penny in the cancer research donation can at a grocery store.
At the root of things, I feel like dishonesty is antithetical to the spirit of hitching. The driver/hitcher relationship is founded on strangers trusting each other; if you fly a sign saying something like "Just showered!" when it isn't so, you're doing a wrong to the driver and debasing yourself as an honest hobo.
Obviously I'm not so philosophical when I've been waiting with my thumb out for 6 hours.