A few weeks ago I made the switch to 100% RO water and have been delighted with the results. My fish are extremely healthy, plants growing well and all shrimp are doing great. I'm even lucky (or unlucky depending upon your point of view) in that something in my tanks, almost certainly the black sand, is raising both GH and KH so I start the week with readings of 4 and 3 and finish it with 7 and 6 so I simply use 100% RO as my change water and it brings it back down.
Personally I wouldn't use tap water at all for any kind of fish simply because the parameters considered acceptable by the water companies are outside of what I would accept as safe for fish in many cases. Also, water companies can change the make up of your water overnight and have do not have to tell you. You wouldn't buy fish, take them home and dump them straight into your tank without acclimatising first and, with the large water changes many of us do, using tap water could be akin to doing just that.
Finally, when doing a recent project in school with my students on water quality we came across statements like "96% of all water supplied is within proven safety parameters". Sounds great at first but I read this as "4% of households receiving poisonous water". As far as I'm concerned our tap water could be poisonous and I won't subject my fish to it - even my Malawi tank gets 100% RO now although it's going to cost me a fortune in salts.