You should rinse your gravel in tap water before putting it inside your tank. I use pasta strainer to rinse the gravel - takes little time with a pasta strainer. I put the rinsed gravel in the tank. Then I pour water into the tank straight from the tap. And I add dechlorinator to the tank once it's fully filled out with water. The reason I do that is that if I dechlorinated the water first, and then put the rinsed gravel into the tank, I'd be putting gravel with
chlorine on it. But when you first put the gravel inside then tank, fill the tank with water, and then dechlorinate it - you're dechlorinating everything: both the gravel and the water. All of this is OK if you don't have fish in your tank. (I know you're talking about setting up your new 30 gallon tank here.)
Once I've put fish in my tank, I dechlortinated the water in a separate container, and then poured that water to the tank. I wouldn't want to pour chlorinated water into the tank with fish. Once you have fish in your tank, and you're performing your regular weekly water changes, in my opinion it is safest to dechlorinate the water FIRST before pouring it into your tank. I hope that helps.