---------- Post added at 09:16 PM ---------- Previous post was at 09:06 PM ----------
Those are all function you can easily do without the toolkit, and I highly recommend doing so (without the toolkit) as the toolkit just adds an unnecessary layer of complexity and additional point of failure.
Relocking the bootloader is a single fastboot command: fastboot oem lock
Flashing recovery is also a single fastboot command (after you've downloaded the recovery you want, and placed it in your fastboot folder on your PC). OK, two commands if you really want to be thorough and wipe cache before flashing recovery.
If you aren't familiar with adb/fastboot, its easy to learn, and quite valuable. Frankly, anyone modding Android phones should know how to use this tool (at least the basics). Using a toolkit to shortcut that learning process leaves you up the creek without a paddle if things go south flashing ROMs, mods, etc (which is practically inevitable).
And I can't imagine why you would need the help of a toolkit to run an RUU. RUU is a Windows executable program. Once you've connected the phone to the PC, you literally just click on the RUU icon on your PC, and the program will launch and walk you through the rest.
The OTA route is very different from the RUU route.
If you intend to return to stock ROM and stock recovery to update by OTA, there is no need to relock the bootloader (and in fact, in this case I do not recommend relocking).
After OTA update, you will need to install TWRP again and flash SuperSU to root. You will need the current version of TWRP (2.8.7); and also the latest version (2.46) of SuperSU, as older versions don't work on Lollipop.
Also, be aware that you need to be connected to AT&T's network in order to receive OTAs on the ATT version. No idea if you are on ATT or not, but I know some folks on here are not (and in that case, RUU is your only option to update "officially").