The answer depends on the age. For preschool age kids, it's not really a big deal with math misconceptions happen, since they're going to be learning and relearning so much anyway, and there are going to be numerous wrong answers along the way. For older kids, you sometimes want to catch this misconceptions earlier before it becomes a habit.
The best way to handle math misconceptions is to try it out and see if it works. This is one reason I like games: if you think you have a winning strategy, play me and beat me! If I win, I've shown you your strategy is insufficient without having to tell you.
But it works for arithmetic too. If a kid subtracts 55 - 4 and gets 15, I have a guess what the misconception might be. I could try to explain, but it likely won't work, since the error is based on a more abstract misconception. So I might reach for another tool (building or drawing 55 and crossing out 4, going to the number line) that exposes the error without it just being me saying "No that's wrong. See what you did wrong here?" Getting kids to understand that something is off with their own understanding and that there is a possibility to fix it is really the key move.