You mentioned hot and cold water supply... if you're using tap water for mixing the salt, use cold water. Hot water (from a hot water heater) normally tends to have more gunk in it due to the hot water heater, and will have a higher tendency to leach copper/lead out of pipes (although very very small amounts) along the way. [That's also why it's recommended to use cold water if you're using it for cooking, boiling pasta, etc...]
If you have a TDS
meter, it's a fun little experiment to compare TDS
readings of your cold and hot water. Depending on your plumbing and age of your hot water tank, you may have very different readings.
[PS... Oh... and I agree that as long as things are covered, it shouldn't be a problem storing water that long. I continually keep salt water stored and only empty the tub every 6 months or so to clean out the buildup of deposits. So some of that water in there, in theory, is 6 months old.]