As long as your water is in the correct pH range and tastes good to you , you can use it. If you wouldn't drink it i would suggest not giving it to your plants.
I use water straight from the tap as my water is always @ 6.0-7.0 ..and my plants never complained. Just be sure to pH adjust the water before adding it to your medium. That's always a must. 6.5-6.8 is where you want it ...when just watering 6.0-7.0 is tolerable.
I've just started using tap water also because they're sucking down a lot of water now. In my house I have a water softener which puts sodium ions into the water and various other things. Luckily I can turn a few valves and crank a knob to get unadulterated water (water that doesn't go through the softener) then I can use that. In my opinion, the extra minerals in the hard water will help the plant so if your house has this, I recommend doing that, then filling up a couple gallon bottles with it. Luckily my plants are grown in the same room that the valves are so I don't have to go far .
BTW, if its the first time you're bypassing the softener or first time in a while, let the water run for 2 or 3 minutes because there will be stuff built up around the valve. If you don't believe me, take a sip of the water that first comes out.
"hard" or "soft" water both are both not recommended for growing plants... the human body can drink a lot more range of minerals in waters then plants.... altho bacteria owns us easier then it does a plant... thas the tradeoff i guess.
But isn't hard water better than soft for plants? I mean, sure it's not the greatest, but I'm just saying what's possibly better. The water in this town is good, so I figured it should be fine for the plants. So far they are doing well with it.
Hard water usually has an abundance of iron and/or calcium... which is good for plants... in smaller amounts... in large amounts it can lockout other nutrient uptake rates.