It is impossible to ruin your knees doing the frog, because there is no external force on your legs below your knees.  Knees are ruined by turning out in the lower leg, especially using the floor to do so.

If you can do the frog so easily, then I suppose that knee-side kind of turnout is something you don't need to worry about.  I would concentrate on other aspects of ballet.  For example, rotational turnout is often harder than knee-side turnout.  Especially with your legs crossed.  What about standing high on your legs in fifth position?  What about maintaining symmetry of your body?  What about smooth movement in adagio?  What about the use and shaping of the feet?  No use in dwelling on the things you already excel in.