Question: When I bought my baseboard heaters, thermostats and 220V breakers I was advised to get 12/3 wire. When I attempted to install the heaters I found that the thermostats, heaters, and even breakers have connections for only 2 wires. I have since read that 12/3 is only necessary for wiring something like a dryer that uses both 220V and 110V over the same wire. My question now is, should I get 12/2 instead of 12/3 to wire these heaters?

Answer: 
I'd use 12/3 -- that way, the neutral is there if you need it (for built-in outlets for instance), and you can use standard color codes. (Just about *ALL* /2 NM is black/white/bare.)
Refer to the total amps for the heater. Base the wire selection on this number. The wire size should be xx/2 WITH GROUND. Try to get wire with black and red conductors for 220 volts. If the wires are black and white the white wire MUST be pemantly marked red or black. Black and white conductors ar ok for 120 volt.