I know that there's a grid that can be used to simplify the placement of objects with seams, but I can't figure out which grid sizes work for what, anyone have any experience with this? Right now my main concern are road pieces.

Well generally you always want to use a power of 2 (2 4 8 16 32 64 128 256 etc) I find 128 works nicely for most interior building, and for roads try like 32 or 16 if they wont snap perfectly together. Just keep lowering it by a power of two until it works.