In general, which is more precise: GPS at walking speed or the Yahoo air photo included on the potlatch edit view? I added some paved trails recently. As you can see from the link, the gps does not match the photo. In some areas it lines up perfectly, so it isn’t a scale issue. I elected to trace the GPS. Should I adjust it to match the photo instead?
http://www.openstreetmap.org/?lat=29.5334547758102&lon=-98.6432281136513&zoom=18
The photos will generally be more precise; although I think an average of many GPS traces over several months is even better. Exceptions may occur in areas of large elevation change (including bridges). Sometimes in mountainous areas major highways will appear ‘wavy’, probably because the photo was georeferenced using an elevation dataset that didn’t include the cuts and fills of the highway.
Not in all places. In my area yahoo has an offset of 1.5 km ! 
Chris