You can root whenever you want, and you can switch ROMs all you want (you just have to setup your phone again every time). You can even try different kernels whenever, but you really need to run them for a few days to get a feel for how they work.
I personally use VaelPak because it's Sense-compatible but has all the Sense stuff stripped out, and it's really fast and smooth. I've had zero problems with it. I used to use KingX's kernel (#11 BFS mostly) and it gave me fantastic battery life, but I also lucked out and got a phone that has always had good battery life even out of the box. It could also be partly my usage habits, and yours will make a difference for you as well. I just switched to Netarchy's 188.8.131.52 with SBC because it was recommended with the new version of VaelPak, but I'm not so sure I want the SBC thing. The whole trickle charging issue is still a bit fuzzy, and exploding phones aren't my cup of tea.
You're going to get all sorts of recommendations for ROMs and kernels, because it's really different for everybody. Sometimes one phone will behave differently than another even with the same ROM and kernel. Sometimes even the same hardware version. Just try some out.
Maybe a dichotomous key would help: Do you like/want Sense? If yes, focus on Myn's, MikFroyo, Fresh, or the other current and actively-developed Sense ROMs. If no, check out VaelPak or, if you want an actual AOSP experience, CM6 or Evervolv. And so forth.
As for kernels, find a ROM you like and then browse through the threads to see what people are liking for that ROM. If you don't like one, try another one until you do. Battery life takes a while to really gauge, especially if you don't wipe your battery stats and recalibrate every once in a while.
As far as I know all of that is generally accurate. And one more thing: ALWAYS HAVE A NANDROID BACKUP!!! Safety first.
HTC Evo LTE S-OFF Running Fresh!