When I came to enter my traces into the OSM DB, I used JOSM and had a slippy map overlay for the Ordnance Survey OpenData. The area I was looking at shows how badly misaligned some of the OS OpenData overlay is. I wanted to add the churches but the (crude) building outlines in the OS overlay were way off according to my traces. This is a common and consistent problem, both with my current GPS and its predecessor which fell to bits. The overlay always shows objects further south than my traces. I trust the traces more than I trust the overlay's position. In the end I traced a church outline to get the orientation right and slid it to a more reasonable place based on my traces.