[Savage GT]

On a painted wood track:
Urethanes leave a fine white powder which reduces grip for everyone else.

Our club has some classes that allow urethanes, no issue mixing. My home track I run a mix of both, no issues.
There is a fine dust produced while truing, its possible they may deposit a little while running (but they don't wear that fast so I doubt it would be much, if any). 
Either way I haven't seen it or felt a lack of grip with rubber after running urethanes.