I like the peace of mind that I get from mixing my own SW. I have heard stories of ppl buying premixed SW from their LFS that turned out to have ammonia, nitrates, phosphates, etc. One poster couldn't figure out where his ammonia in the tank was coming from. He mentioned using premixed SW. Someone suggested testing it before adding anymore to his tank. Sure enough, the premixed SW was the ammonia culprit. However, I would definitely trust a LFS' premixed SW over Walmart's RO water though. Someone did a TDS test on Walmart's RO water and the TDS were pretty high which would lead to algae issues. Your TDS for pure water should ideally be 0ppm to avoid algae problems. Of course, you can't test TDS on premixed SW since it has salt in it. The test is for pure water prior to adding salt.
And even if you bought premixed SW, you still need pure top-off water. So either you are also buying pure RO or RO/DI water for top off in addition to the premixed SW or you are doing a major no-no and adding tap water. Or you have an RO/DI system to make top off water but still buy premixed SW for some reason. Anyhoo, an RO/DI system will pay for itself over time. No lugging buckets full of water around with the possibility of spills in the car, etc. You know exactly where your water came from and how old your filters are. You can make a bucket while watching TV or fooling around on the internet. A win/win situation, IMHO. And it is fairly easy to get a good RO/DI system cheap. I see used systems for sale frquently in the Lexington area from ppl who are leaving the hobby. Lots of these ppl were only in the hobby a short time so pretty new RO/DI systems are often being sold for major bargains.