Is it safe to cut through all the wires at the same time in a 14-2 romex wire while the main breaker is shut off?

If you're ABSOLUTELY CERTAIN that the breaker is off... no problem. Cut them all at once if you like.

However, just as many people have been shot with an "unloaded gun", many people have been electrocuted by a circuit that was shut off.

If you use lineman pliers that have an insulated handle, (don't touch the metal part) and the somehow the breaker DIDN'T get shut off, you'll be startled, the lineman pliers will be destroyed and there will be a puff of smoke to let you know that it was the"other" breaker.

As always, if you don't know exactly what you're doing, hire a professional.