Tap is perfectly fine to use. I got suckered into running R/O water in the beginning. It starts to get a little costly so I went to tap. I figured if you can water house plants, as well as any other plant in the world with tap water and have no ill effect then it's not going to hurt the ones you are growing. I even started letting the water sit for 24 hrs to let the chlorine evaporate out, but now I don't even do that. I just hook the hose up to the faucet, put the other end in the res, turn it on and fill the res up. Haven't had any problems with it, and it saves me a ton of money and time. The very small amounts of chlorine that's in the water won't hurt the plants or roots. If anything it may, and I do say MAY, help keep any nasties away from your roots, but I haven't seen any scientific proof that it does, but I just like to believe it can help a little bit. Sure, they say no to use beneficials if you don't let the chlorine evaporate out. I'm sure that is true though, that's why I wait about 2 days before putting any beneficials in the res, just in case the chlorine does kill them off, and those benies sure aren't cheap.
Don't buy the hype of R/O water. The only thing that is good about R/O water, to me, is that it's a clean slate, and you know exactly what is going into your res. That part is nice though. Using R/O water they say you need to add Cal/Mag, that's another thing that you have to buy and waste money on since tap water typically has enough of both for your plants. So you don't waste money on buying cal/mag all the time.