How much seam allowance should I have for drywall joints?
When hanging drywall material it is not necessary to leave a seam allowance between sheets. The main concern is to make sure that pieces fit up together without forcing into place. Forcing can cause edges to breakout and create an uneven surface when you come to do the taping. 

I just also checked a SheetRock brand installation guide from USG for their gypsum wall board materials and they also are not recommending a "seam allowance". 

On the other side of the fence you want to avoid leaving large gaps between sheets as well. These can be really problematic to get good results trying to bridge large gaps with tape and joint compound. My recommendation is to have no gaps larger than about 1/4 inch (5mm).

An exception can be made for certain types of joints that are to be trimmed over with edging. For example on an outside corner or in a banded door opening it can actually be an advantage to hold one corner of the drywall from the overlapping side as shown below. The space left makes it easier to line up the edging material on the corner when the walls and/or framing are not nice and true.