We didn't get a transmitter for the toad. If/when I need a spare tire, I will check the pressure prior to install. If low, then I will air up with on board compressor. I do periodically check the spare pressure as well, but except for gradual air loss, the spare should not need anything more than a top off, if that, when needed.
Since I have the capability to air the spare up prior to use, I'm not worried about it being low until/unless it is used. At that point I will put the transmitter from the tire I removed onto the spare.