First off you really do not want to use straight RO/DI water, unless there is something really wrong with your tap water. It would be ok however to mix half tap water & RO water, this way your fish & plants will get some needed nutrients.
Each setup is different so the ideal water parameters for one setup is going to be different for another, if there is such a thing. For PH that will mainly depend on the fish you keep. Most fish do well between 6.5-7.5, for KH/GH between 3-10ppm & C02 levels between 20-30ppm.
Trust me, the amount of sodium and chloride and bicarb in my tap water is really bad. It also has a really high content of particulates which makes water cloudy, so it has to go through a sediment filter anyway. And I've already bought the RO/DI. It's no problem reconstituting it with chemicals for me, I just need to know the parameters I'm shooting for.
In my 50 gallon tank (42 gallons actual water) it's about 1tsp baking soda for +1dKH increase in carbonate hardness. I put in 3tsp tonight and will test the actual value when I get a test kit (I've got some test strips but the kH accuracy on them is bad). I've ordered some 'ultimate gH booster" from Greenleaf Aquariums and it's 2:K2SO4 + 2:CaSO4 + 1:MgSO4. I'm sure the MgSO4 is heptahydrate (epsom salts) but I don't know the hydration states of the other two so I can't calculate how much to add. I guess I need to give them a call or an email.