it really doesnt matter if you trun the lights off when you are treating for ich, unless you are doing a total blackout for green water. you should do everything else normally, because the fish are already under stress and dont need any more surpirses.
Table salt is a whole lot cheaper too. If you run out (or drop your container into a bucket of water - don't ask... ) at an inconvenient time you can get it anywhere. Even a 'convenience store'.
If it makes you feel better, you can get non-iodized. Some stores carry it - it's mainly for people with certain Thyroid problems, so it may cost a bit more, I dunno. The trace amounts of iodine won't hurt your fish. Neither will the trace amounts of anti-caking compounds.