When I began sewing garments, I pinned everything. After a 4 month stint in a sewing factory, I could sew plaid pants, matching patterns, without pins!
Basically, I pin if I think something needs to be precisely matched. If there are seams that butt together (like 4-patches, etc.), I don't pin. For borders, I fold the top and border into quarters and pin at those points, which gives me room to ease the fabric. If I'm putting sashing on a block, I don't pin, but if I'm putting sashing on a row of blocks, I treat it the same way I do the border -- fold it in quarters and pin. I don't pin when I'm chain piecing or when I'm putting sashing on individual blocks. And I always pin curves.
Elizabeth