Well if you're going to use CR2s, make sure they're rechargeable 3.6v protected batteries. See if you use two normal 3v CR2s or 3v rechargeables you'd have to constantly buy or recharge them. The LM317 takes 3v, if you power your diode at 250mA that's 2.8v, so in total you need 5.8v to power your laser, and as soon as the voltage from the batteries drops below 5.8v your laser will start getting weaker and weaker. So like I said, if you use 6v only you'll have to change or recharge the batteries constantly. Not to mention if you included a diode in your circuit to protect it from reverse polarity, that's an additional .7v so now you'd need 6.5v to power your laser which means that by the moment you put brand new batteries in, the LM317 would start dropping out immediately. So in conclusion, just use 7.2v to power your laser, if you can't find a power source of that voltage that fits in your host, than you'll have to change to another one.
PS: Make sure the power source you get is at least 500mA, otherwise even if your source measures 7.2v it won't last that long.