_IF_,
Keep in mind, X10 is not the most robust PLC communications protocol out there, and it's not a closed loop system (meaning that commands are merely tossed onto the wire and are not acknowledged). So, if your heaters are on, and the off command doesn't make it through, things could get quite toasty! You'll want to make sure you embed retry logic into your events to resend the command periodically, or use a technology that does support ACK of commands and query status commands (INSTEON, UPB, Z-Wave, etc).
Most of the home automation lists I'm on recommend that you NOT control anything with X10 that can burn your house down if the off commands don't make it through; similarly, they don't recommend it for cases where a life could depend on it (your reptile cage, fish tank, etc).
HTH!
/Mike