RO water is still a good choice for extract brewing. All the minerals from the mashing water are in the extract, so you don't really have to add more (that assumes the maltster's water had enough calcium). The low alkalinity of RO water also means that you run less risk of extracting tannins or silicates from your accessory grains.
Its a win/win for extract brewers. When you go all grain, start at the Water Primer mentioned above and work with that until you are comfortable. Then you might venture into more elaborate water chemistry efforts with a program like Bru'n Water.