Where a Seam Should Fall
Seams Are Not Avoidable, Only Placed
Slabs come in finite sizes, and any run longer than that has a join. Access constraints can force one on a shorter run too. Since a seam is going to exist, the only question worth asking a fabricator is where they intend to put it and why, which is a conversation that belongs at templating.
Never Through a Cutout Corner
The inside corner of a sink or hob aperture is the most stressed point on a worktop. Placing a seam so it runs into that corner combines the two weakest features in the same place. Reputable shops treat this as an absolute rule and will redesign a layout rather than break it.
Over Support, Not Over Air
A join needs something solid directly underneath, and appliance apertures are the usual offenders because there is nothing there by definition. Where a seam has to cross one, a steel or timber rail is added beneath. Ask whether that is in the quote, because it is the sort of item that gets discovered rather than planned.
Using the Room to Hide It
A seam at the centre line of a sink is conventional because the eye is drawn to the bowl and the join is short. A seam at an internal corner works because the direction change breaks the line anyway. A seam in the middle of a long clear run in a bright kitchen is the worst case and the one worth paying to avoid.
Pattern Continuity Costs Material
Carrying a vein across a join means cutting the second piece from the position that continues the pattern, which usually wastes stone. On a plain material this is irrelevant. On anything with movement it is the difference between a join you notice once and a join you notice daily.
Judging the Finished Article
Look along the seam with light raking across it, not from above. It should be flat to a fingernail, tight, and filled with colour matched adhesive rather than a generic clear. A step between the two pieces is a levelling failure. A visible gap is a cutting failure. Both are worth raising in the first week.