Yes you wanna make backups anytime before you flash something, also a good practice to make them randomly just in case
You may also wanna check out moped's thread for the data fix, the fix is actually now included in cm11 builds but for my personal device it seems to work better when I use moped's fix
If you like customizing colors and such you can check out the xposed framework and the xblast tools module for it but they have specific directions that must be followed to the T
Also so your aware the cm11 has 2 normally used types, snapshots or M builds which are considered the most stable and then nightlies which are basically them making little fixes and such working to a new snapshot, that's why you'll see night lies having issues like GPS not working correctly
Also along with the ROM itself your going to need a gapps package in order to get google play working, I use paranoid androids gapps but there are a few out theit,
And you also need to be sure you have the sph-l710 version of the phone cause there are virgin mobile s3's that are sph-l745 which I believe are Canadian versions but I don't believe the 710 Roms will work on it, but its been awhile since I've seen anything on that device so I'm not 100% on that
If I recall correctly this its basically
Sph-l710= sprint/virgin mobile USA variants
Sph-l745=att/virgin mobile Canadian variants