"Does it really matter? How long after a meal should we leave it?"

Yeah, it matters, especially if you eat heavily. And yeah, the answer to "how long" is learned by experience.

Basically, you shouldn't be hungry and shouldn't be stuffed. If you are doing deep breathing with full chest expansion, full diaphragm distension (pushing into the stomach area), etc., then a full stomach makes it literally impossible - there's no place to push.