Water questions?

Hello, there is a theory that you should let tap water sit out for 24-48 hrs before using it, my question is if you let it sit out covered or sealed (I.E. old plastic milk containers with the cap on) is it doing any good, or does it need to sit uncovered? newbie thanks!

I fill my watering can 24 hours prior to watering the girls. Actually I fill it right after I water the girls so its all ready for the next watering. So it sits out for 72-96 hours each time.

It definitely has to be open to the air for the fluoride & the chlorine to evaporate.

Thanks for your quick response GREAT! the last 9 months of my growing experience I've been storing it covered!! I've been doing it wrong the whole time!lol!!
funny though, the plants came out well! makes me think if theres anything to letting it sit out?