Should I use a graft or a seam when attaching sweater shoulders?
Seam. The shoulders of a sweater are a high-stress area; the entire weight of the sweater hangs from them. A nice, solid seam gives a sturdy structure for that hanging. A graft - which is designed to be stretchy, just like the main fabric of the sweater - can result in droopy shoulders and stretched necklines. The excellent Techknitter wrote an illustrated article about the importance of seaming sweater shoulders, with lots of detail and tips for different types of necklines.