Well first thing I will say is - eh root the phone, if you need to return you can unroot it, if it gets to the point where you can't unroot it then they are not going to figure it out till you swap it. As it stands I rooted my phone and had to have it replaced......I sweated they would charge me even though I unrooted it (although you could probably figure it out it had been if you knew what to look for). When I took it to the store we fiddled with a few minutes and told me to come back Tuesday to swap it (I was having a weird network problem where I could to the network in some places but not others, and I'm talking I could identify when and where the phone would pick up the network, and where it would drop it). In the intermittent time I rooted and did an even lesser job of un-rooting it went in, they didn't even look at it. I believe we just did a data wipe (or maybe I did that on my own, don't remember) and put the microSD in the new phone. This was a corp store to boot. Your decision but I wouldn't sweat it and let it stop you from using your phone the way you want.
That said the best solution for the battery is an extended battery. I'll recommend this one because I am very pleased with it (I've written a few positive reviews on a number of threads) and had good support from the seller (I made a wrong click on ebay and had to contact him to fix it which he did with no trouble). Anyway, in my opinion that is really the best and only fix for the battery life.