The only purpose of scribe lines is to register the chisel to provide a clean and straight line of chopping.

You can't do hand cut DT's without some way of registering the chisel.  

Usually in drawer making the drawer is made tight and planed to fit, and the scribe lines will be removed in the process.  Some people chose to remark the scribe to indicate hand cut.  

You don't have to scribe across the whole board.  You can either only scribe where you will be removing or you can not scribe at all, and clamp a ruler across to register the chisel against.