When hanging drywall, how much distance should I allow for seams? Do pieces fit flush against each other, or do I leave a gap? This is for USA construction, in case there is some code for such consideration.
When measuring, I tend to assume the seam between drywall and the adjacent surface (wall or other piece of drywall) will eat at least 1/8" from my measurements, and sometimes 1/4". You're not intentionally leaving a gap, so adjacent pieces are pushed as tight as you can against each other (unless you have a gap to other pieces of drywall on both sides, in which case you center it). But with rough edges, you have to account for this extra space to avoid forcing the drywall into place. This impacts you differently depending on where you're measuring:
When measuring for cutouts, that extra offset can sometimes be added (near side), and sometimes cancel out the original offset (far side), so be careful to double check your measurements (or use a rotary cutout tool so you don't have to measure those with any accuracy). Remember to transpose your measurements, especially for ceiling cutouts, since the drywall will be flipped from resting on the ground to being suspended over your head, where left becomes right. I often mark two sides that all my measurements will be based off of first so they don't get mixed up.
When fitting drywall into an opening (the last piece that has to go in) you have to deduct for both sides. I'll start with a 3/8" deduction and be ready to rasp down any high spots. Always measure both sides of the opening (4 measurements, not just 2) since things often aren't square.
On the ceiling, you've got the drywall on the wall to hold up the ceiling and cover the gap. You just want the gap to be under 1/2". I would subtract 1/4" from a measurement to a wall in this situation.
When measuring for walls, it's mainly for the vertical joints to the next wall, and those gaps are often covered by door trim or the drywall on the next wall in a corner. Drywall is installed to the top of the wall first, supporting the ceiling that was already installed, and then it's installed on the bottom half of the wall. The gap around the floor can be at least 1/2" and is often closer to 1" and will be covered by trim.
When measuring for outside corners, don't leave any allowance. Instead, use a rasp to knock down the edge until it's perfectly even with the adjacent wall.
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.