Question about slicing the top of a pork butt...

I'm going to take a shot at this even though I'm not 100% sure of exactly what you are asking. However I think that you are asking about scoring the fat cap. If that's the case there are a couple of schools of thought on this. First it does allow heat to the lower part of the cap much faster thus aiding in rendering down a real heavy cap. Secondly if you score all the way through the cap, some folks like to put rub in this slits. By doing this you are actually putting rub right on top of the meat that is under the fat cap. Me personally, I see little benefit from scoring meat that will be slow smoked. Nothing wrong with doing it, I just deem it un necessary more often than not. Now scoring a world famous flounder from the Gulf of Mexico before deep frying, well thats a whole different ball game my friend. I highly recommend scoring in this situation. I hope that I at least partcially answered your question my friend. If not, I will gladly try again.

More surface area for the rub.

I normally remove the fat cap and expose the meat. Put rub on the meat and create more surface for bark. Also less fat collected in the drip pan to clean up.

Here's my experience with butt fat cap. It just falls off after the cook no matter what. So either I trim it off and get more bark or I just do fat cap down and let it peel off anyways. I'm doing two butts right now (letting the egg stabilize as we speak) and I wasn't feeling like trimming off the fat cap so I'll go fat cap down.