A problem with excess fabric, even after measuring, might be if strip lengths are cut WOF where there is more stretch in the weft as opposed to Length Of Fabric where the warp is taut.
When I add borders or sashings with or without cornerstones, I consider what should be (if there are 12 inch finished blocks plus "x" number of sashings each of "y" inches width) and then measure what I've got. Hopefully they are exact (Haha) or within a 1/2 inch. I, too, pin like crazy, but divide and mark the quilt in half and then in half again (fourths) as well as mark the added strip (sashing or border) so that the marks (fourths) match before I sew. If there is any fudging to do I can adjust by slightly tugging/stretching the quilt or strip. Measuring and marking and pinning at the corner fabrics also helps. It takes a bit of extra time, but well worth it!
I hope that all makes sense.
Good luck.