Well don't wait until a centimeter! The further they get out of true the harder it is to true them. And it is not a linear relationship - when they are far out of true you have to worry about dish. When they are slightly out of true minor tweaking will get you back. As soon as I can detect they need trued while coasting, I will true them. That is usually less than a few mm.

YMMV but I find a tension meter a good investment if you need to do this often. When you have a wheel that you know is true, tensioned right and dished correctly, you can measure and record a tension measurement for each spoke for reference. Takes hardly any time. Then you can use the average as an aid for deciding to loosen one side or tighten the other for high or low spots.