I get fog problems when it's unexpectedly warm (usually in the teens or twenties) and I have to stop for very long. Then evaporation from the sweat will fog them up. Otherwise fogging hasn't been a problem. Any wind at all, or just riding along, will keep them from fogging.
A bigger problem I ran into was freezing eyeballs after downhill stretches. My left eye froze shut last winter and got infected afterwards. It cleared up, but was disconcerting to say the least.