Is that a general rule of thumb?
We must have different thumbs.
There is no specific study on serotonin receptor density with methylone that I can Google.
If there were, I could give a better answer.
With MDMA, we can say that receptor density is back to reasonable control within
21 days after a very heavy dosing schedule. (With rats, of course, which are different than humans... hence the once the month and occasional extended break rule that plays out in practice with humans. Not a hard, fast, rule of thumb... but one that seems okay enough.)
With methylone, we don't even have that data point. At least that I can see.
Methylone *feels* less neurotoxic and more repeatable than MDMA, in part due to the less potent SERT release. (And, SERT *seems* to be slower to recover than DA or NE. Insert depressing factoid about humans being more adept at fight-or-flight NE or primitive rewards DA vs more seemingly complex emotive state transport systems like SERT, a tangent for another day).
However, I will make no conclusion personally based on "feels". If you want to be the human guinea pig to prove there is no cross tolerance, feel free. All I can say is that in lieu of the science we have a few things in Erowid and elsewhere that does demonstrate some cross tolerance between methylone and MDMA, at least anecdotally. The degree, I cannot say definitively, but it's better to be cautious.
Feel free to experiment if you want to expand the safety definition. There is not a lot of hard data and a lot of this is playing it safe in lieu of it.