Nice, so then you can download several roms, and with rom manager just tell it you want to use that one and it will reboot into that rom? So if a rom requires date/cache wipe the rom manager takes care of all that as well?
I'll answer your first question about being safe with that back up: Possibly.
If you did a nandroid backup with SP Recovery (Which I assume you did 'cuz that's what you had when you hit that point of the instructions, just like I did) and you still have SPR installed, then yes, you *should* be good.
If you have installed another recover console, like ClockworkMod from ROM Manager, then no, because the two are not compatible.
I do agree that you should get ROM Manager, it is *well* worth the money because it makes your life so much easier, you don't have to mess around with command-line tweaks or manually using the recovery console unless you need to recover from a failed setup (I did it earlier today, flashed a ULV kernel that put me into a boot loop, pulled the battery, went through Clockwork to one of the other kernels that I downloaded previously that I knew worked, and I was golden!).
It will let you easily change ROMs as you want, which is really why I love it. I bounce around and try all the ROMs until I find one that I really like, but until then, I may end up flashing my phone to different things 2, 3, even 5 times per day, and ROM Manager + Titanium Backup make the transitions smooth as silk