Ha!! Already postin the questions huh? Crazy kewl.
Like Lucho said, it really depends on what species you want to keep, how large your tanks are, and the like. I only used RO water towards the end of my stay in Nebraska, but I could definately see the benefits of it. Of course, I kept mainly SA cichlids and the like. If you plan on keepin heathen African cichlids then the tap would be PERFECT for them...as it is relatively hard and alkaline. Most fish species will adapt to the parameters of your water though, so its really not neccessary to change it unless you have intentions of breedin certain species.
Generally, SA (South American) cichlids, Characins (tetras, wolffish, piranha), arowana, gourami, loaches, and the sort like soft water, acidic water. CA (Central American) cichlids, some barbs, African cichlids, and a whole pleuthroa of other fishies like hard water, alkaline water.
Since you've got access to 'free' RO water, might as well take advantage of it! You can do like he said and do 50/50 with tap, or 75% RO water and 25% tap. OR you can just do 100% RO. All you would have to do is get some old panty hose, cut one of the legs, and fill it with CRUSHED ARGONITE CORAL. Tie it off and stick it in one of the filters or somewhere unnoticeable in the tank. This will help buffer your tank and keep those dreaded pH crashes from happening. HTH a bit....