On my ML, I had hastily screwed the gas cap on after a fillup and got the check engine light on. I was dumbfounded until my wife (who listened to the "new car" audition) told me that I probably did the cap wrong (needs 3 turns until you hear that awful ratchet-like sound). So I went outside and re-did the cap properly. Check engine light was gone after that...no resetting necessary.
On other cars I've owned, the "check engine" light was always a bad O2 (or lambda) sensor...the only immediate problem would be if your car is up for an annual inspection, in which case, it probably wouldn't pass.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car