Should you draw out your cuts on your tesserae?
No, not ever. It may be different in modern mosaics where you want a very specific cut or where you want no gaps between them.
In making a copy of a Roman mosaic you need to develop your eye to make the correct cut without these marks. You will only do that by making these cuts, getting it wrong and working with the material to figure out how to get it right.
Another reason is that, whatever the size of your mosaic you always work as if you were doing a full floor. Stopping and drawing on tesserae for your keystone cuts etc will just waste too much time.