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.)
I wire both of my 240 heaters on the same circuit with 12/2 wire identifying the white with black electrical tape.A 240 circuit is alot cheaper than a 120 volt.