Some breakers are rated to be used as switches, but residential type usually
are. Doing once in a while is probably okay.
I put a switch on my cottage water heater, and I put it in a more convenient
place than the circuit box. Assuming access isn't a problem it is simple
enough, but if I had it to do again I would have used a relay and put the
switch at the front door.
Turning off all the power is fine, but probably unnecessary; I don't even do
it in the winter when I won't be there for several months. (If there is an
electrical problem, I would rather fine out when I am NOT there...)