Should a heavily loaded drawer be mounted near the bottom of the rack?

On a small free-standing rack, yes, that is a sensible precaution.

Pulling a loaded drawer forward moves its centre of gravity towards you.

A substantial 42U floor cabinet or a properly wall-fixed cabinet is a different proposition from a shallow 15U free-standing rack, so there is no useful universal height rule.

Where stability is in doubt, keep the drawer lower.