JRein,
I have direct experience with Wal-Mart water so I though I could offer some input...
The Walmart by me sell 1 gallon bottled jugs of RO Water (its a white and green label and says "drinking water" but a closer look says "Reverse Osmosis"). They also sell another, purple label, that says "Distilled Water." I think this is what you're talking about.
I've been using this for top offs and water changes for almost a year now. Aside from the 25 1 gallon jugs I have to lug out of there each month, I've also found something else:
For some strange reason my PH has been very high (8.6-8.8) the past 8 months and I think its the Walmart RO water I use. Try it yourself before using alot of it. I tested the plain water right from the jug and received a PH of like 7.6 . After I added Seasalt from Aquarium
[hr]
(respectable and widely used) the PH jumps to the 8.6-8.8 range! I cant explain why. I even contacted the nice people at Aquarium Pharm and bought a new batch just to make sure. Whatever it is, I have come to think that its something in Walmat's RO water. I would love to hear if anyone else has founf this, too.
So, bottom line, my opinion is that it seems to be great except for this strange PH thing. I would just test a batch of it, mixed with your seasalt first.
Hope that helps!