How much shaft should be sticking out in front of the riser @ full draw or does it matter?
Enough so you don't accidently cut your finger on it or pull it off the string on an overdraw situation. I leave 3/4".
I cut my arrows to 29" or 30" for my 26" draw.
I leave mine full length(32in) I draw 31.
I just trim mine untill they tune with the point weight I want to use.They run anywhere from 28 to full length depending on the spine arrow I want to shoot.The length does not matter unless it is too short.
Anything past 3/4" beyond the back is there for some other reason than necessity.
You would want enough clearence for broadheads.But on carbons I let the bareshaft tuning determine the final length of the arrow.Carbons don't come in as many different spines as wood or aluminum,so I trim and tune from full length and the length they end up at is where I leave them.With carbon arrows I don't worry about their length.
I'm with pat.29-30: also helps with getting the right spine. You can get a arrow that is just a little stiff for your bow when cut close to the draw length, but being a little longer you get more weight and better flight.