After having a tummy tuck, is it safe to wear a waist cincher? If not, how long should I wait?

That is a question better asked of your surgeon. He/she knows exactly what was done during your surgery and how your recovery is progressing. I would think it would also depend on "how" it fits. Smooth distribution over the entire area of surgery/swelling would be far preferable to something shorter or les even in pressure.

All your postop questions really need to be addressed to your chosen surgeon, Dr Robinson>>>>>>>>