While struggling through an etude tonight, I noticed a big difference in tone between the parts I knew well and the parts I kept fumbling. The known stuff sounded nice, while the tone in the not-known stuff was simultaneously whiny and rough, as though in sympathy with the struggling left hand. When I made an effort to fix my tone, the whole thing (left hand included) started to sound better. I know this sounds like "I sound better when I sound better" but that's not what I mean.
I think the secret to everything is: you can be forgiven a lot of wrong notes if you make a nice noise.