It is sad that contractors spread this sort of nonsense. Cupping and warping are not a mystery.

If:
The house is stabilized for humidity and temperature,
the wood is appropriately kiln-dried,
allowed to acclimate in the home,
installed when both flooring and subfloor materials are the correct moisture levels,
and moderate humidity levels are maintained in the home year round,
Then your flooring should be fine.

All of those steps should be taken whether your flooring is 3" or 12". My floors range from 3-12" and they are fine. That said, if a customer lives by the beach and prefers to leave the house open to humid air all summer, and then use a humidity-sucking woodstove all winter, they will likely see some movement.

Thanks! Thats pretty much the info i found online, exactly how you stated. Should be fine - im going with what i want!