Unless you are planning to grow/raise something unusual the hardness is not likely to matter to much. Here is what Tom Barr has to say about tap water and hardness;
Using tap water
Tap water is cheap and water changes take less time than the testing (salt water is the exception perhaps, salt mixes cost a fair amount money). Water changes also cost less than test kits/testing and are more fool proof method of estimating the nutrient levels in your planted tank when dealing with NO3, Fe and PO4. It's also simpler and requires less knowledge of chemistry and testing against known standards. Plants are most often starved of nutrients and inaccurate test kits are largely responsible. Many people feel tap is unsuitable for plants, this is simply not true. Old myths still abound claiming excess PO4 in tap water causes algae, this has clearly been shown by many hobbyist to be patently false. The tap water has nutrients in it, then you do not have to dose these nearly as much, this is actually a good thing! Why take something out and then add it back again?
Have hard water?
Great, you do not have to add any baking soda and GH builder to your tank. Adding enough GH to bring the levels to 3-5 GH degrees will address higher light tank needs over a week's time. You can use SeaChem Equilibrium for this or a mix of CaCl2 (or CaSO4 although it is not as easy to dissolve into water) and MgSO4 at a 4:1 ratio to increase GH. You can add this without knowing what your GH is by adding 1 degree's worth after a weekly water change (or slightly less with less frequent water changes)
Plants prefer soft water? Not so, neither myself or other experience aquarist have found plants that are soft water dependent, although there may be a few exceptions out perhaps 300 species, it is safe to say that plants prefer harder water and there is research to show this is true, (Bowes 1985), (T. Barr, C. Christianson observations of clear hard water springs in Florida, USA and in Brazil). A few plants, about 5 or 6 or so species do seem to prefer softer water, but this is due to KH, GH seems to have little bearing as long as there is enough Ca and Mg. So the GH can be dosed a little higher if in doubt or if you want to check to see if that is causing an issue or not.
KH on the other hand does seem to influence these specific plants(most are not affected) to about 5-6 degrees. There is really no limit on how low the KH can be for good plant health, but it can make CO2 measurements trickier. There is a way around that though. Still, any plant can be grown at a KH of 5 and a GH of 5-10, or less. This would not be considered "soft" water, actually it would be ideal. Thus unless you desire to grow a few eclectic species, there is no need for RO, nor DI, carbon filtration of the tap water, but doing so will do no harm to the plants as long as there is enough GH for the plants and KH to determine CO2.