To back up full rom..like everything,as the post says above you would boot into recovery and back it up that way at the moment due to not having official clockworkmod..dont know if it is yet,but you can do that way.itll save everything just as you left it.
titanium back up will save apps and there data..
and can save the way you have saved your settings etc.
itll save your profile without needing to login.then click on options..then batch if its apps.or if its system data and apps choose that option.
beware though if you got 100 apps youll be there for a good ten minutes reinstalling lol.
(note--asop isnt around yet fully but dont restore system data from sense roms to asop and vice versa itll just mess your system up)
your external storage( will never be touched with relation to flashing roms etc.the only way it can happen is if your a complete numpty and select that option yourself.
best way to be honest is just play around with titanium youll get the hang of it
By the way the thanks button is there for a reason,shame they dont have a 'jog on' button either