But if u rooted it then u hv to unroot it first
Not needed .
There is a loophole in the technical warranty .
The reason rooting is not allowed is because it means changing in /system and much dangerous changing/flashing the nand .
ROM Manager bricks the Bootloader of the phone .It's a rooted app that does it.
However the bootloader can also be bricked in other ways
A)A bad odin flash (Only if odin is halted while bootloader is being flashed ).Odin is used by samsung personnel .
B.)KIES OTA failure .KIES has a very high failure rate and it's an official application ,so technically you are under warranty .
After the bootloader is bricked ,no one can know whether a phone is rooted or how a phone bricked (Odin,ROm manager or KIES ) .
So they just restore the nand and flash a stock firmware .
Samsung Care Centre don't care if you rooted or not .
It's just a disclaimer added to discourage people from doing stuff they dont understand (Like flashing Clockwork Mod recovery meant for other phones )