Trust their experience. They are the ones providing warranty and are the ones who don't want to get called back to fix anything. Often middle of sink doesn't have the cabinet support for the heavy counter. That's why they often choose to move away from sink, so the counter has the strength to carry sink.

When I met with a fabricator and asked the same question, she said she preferred the seam at the sink whereas her boss preferred it elsewhere. She liked that the seam at the sink was only 8 inches of seam whereas her boss said that seams at the sink have the highest probability of staining because of water being in contact with the seam all day long. This is a bigger problem with light colored seams. Strength was not a determining factor.

When a sink is strapped in place, the sink becomes a 10" deep truss, turning one of the weakest areas into one of the strongest. No need to fear putting a seam here.

Put the seam at the sink.