Until five minutes ago when I googled it, I didn't even know that the little rectangular blocks of wood sometimes placed at the intersection of architrave and skirting had a name- turns out these are 'plinth blocks' or, if decorated with some sort of detailing, 'rosettes'.
I'm planning a fairly contemporary finish and going to use primed MDF which will be finished in a gloss white (to give a slight contrast to the matte white walls). I *think* I prefer the look of plinth blocks, but I know you can go ahead without them.
I also wonder if using them makes fitting the arc/skirt a tiny bit easier?