I have been doing kickboxing for a year and mma for 6 months now. Before I started kickboxing I spoke to my optician about this very situation and was told that as long as I am wearing soft contact lenses and not hard ones I don't need to worry about doing contact sports. I have had 3 kickboxing fights and have not had a problem in the fights or in training. My last fight I got cut above my right eye and the blood didn't cause any problems with my contact even thou the blood was running straight into my eye.
Over the last six months I have averaged 5 hours of kickboxing and 3 hours mma training and have not had a problem as of yet.