The only case where you wouldn't want to use the standard port configuration would be in those very rare few regulators that may restrict one of the low pressure ports to limit flow to a BC etc. You wouldn't want to use that port for breathing. The only reg like that that in my memory is some early Poseidon regulators. They marked that port for the drysuit connection. Otherwise, the regulator is gravity independent and you can orient in any way that gives you the best hose routing and access. With Tech diving, one of the regs is almost always in a "non-stock" orientation.