i suggest you do the process anyways.
im not sure if you will loose data (cant remember), but there is a risk as in everything involved to flashing.
in case you do loose data, to loose the least data possible, back up your SD card data on your computer. i also suggest you sync your contacts to your google account. once you have done that, you will only loose apps, app data, call logs and messaging history. you can redownload all the apps later, so you would come up loosing only your logs and app data.
i recommend you also get S-off (the wire trick is easy and fast), and downgrade your Hboot. because with S-On flashing custom Roms is somewhat of a pain. and with S-On you cant downgrade Hboot, and if you dont downgrade your Hboot AOSP Roms wont work. only Sense Roms.
im willing to help you if you are having trouble with the guides online.
also, is your Phone GSM or CDMA?
Mine is CDMA. i did everything (rooting, s-off, downgrade hboot, flash recovery). and it was pretty simple and quick. but i did loose my data. my SD card was untouched though
also, you cant do a Nand backup unless you are rooted at least.
but, after you are you will never loose data, as long as you make a Nand every once in a while, and back up your apps using Titanium Backup constantly if you decide to flash roms often.