In this question about using Liquid Plumr or Drano, I'm not sure how to handle choosing the accepted answer. I had already accepted an answer before another answer was posted. Both of these answers seem to be valid but somewhat contradictory to each other. It turns out that the most popular answer is the one that was posted after I had accepted the current answer. At this point I'm not sure which one is "correct". Do I leave the currently accepted answer even though most people seem to disagree? Or do I not accept any answer at all?
The universal SE guidance is to accept the answer which worked for you. If you believe the claims by the manufacturers that the products are safe for your septic system, by all means keep the currently-accepted answer marked as accepted. If you are swayed by the popular opinion of the other answer, mark it as accepted instead. If you aren't sure, don't mark any of them as accepted.
The votes tend to bubble the "correct" answer(s) to the top (correctness is sometimes less black-and-white here than it usually is on SO), but there can be more than one correct answer. The checkmark is just to show which answer worked for the OP in that situation.