No, it wouldn't be a good idea. Pure Ro di water has pretty much no buffering capacity, so you would be having major problems with the pH level. There are also some trace elements that the Betta will actually need that won't be present in that type of water. And, if you have live plants, they will die in this type of water.
Possibly, you could use Ro di water and then add Seachem equilbrium to help solve those problems.