Legacy/ TMCC locos powered up via a toggle is just fine. Its no different that flipping the switch on a Lionel 180w powerhouse. I've been doing it for 16 years, haven't had an issue yet.
I have toggles going to isolated tracks anywhere a locomotive might be parked, i.e. siding, yard, etc.
This also prevents trains from entering blocks that are powered down in the event the turnout is accidentally set to diverge.
Likewise you won't have a train sneaking out of a siding if accidentally addressed either.