If your res temps are at 66 I don't think you need a heater. The biggest issues folks have running DWC is keeping the res temps below 72, which is ideal. Your lower temps are probably what's keeping away any slime issues etc. I've struggled with keeping my res temps down and have made it a practice to use ice water when changing out my res. Sometimes I will put some ice packs in the res or frozen bottles of water, but that only works for a few hours
I top mine off every day. I have found that by doing this, I can get a handle on whether or not the plants are eating, drinking water or both and then I top off based on that. If my PPM goes up, then I conclude that my plants are eating more than drinking, so then I top off with ph adjusted water. If My PPM goes down, then I conclude that my plants are eating more than drinking so I top off with nutrients that I have in a gallon jug made at the same concentration levels as the res. If it stays the same, I add a little of both as the conclusion is that plant is taking in relative equal levels of water and food. Basically,I pretty much keep my water level right at the bottom of the net pot for the entire grow.
Do you plan to switch from MH to HPS for your light?