A friend just asked me to look at a vest she had knitted. Most of it was just fine, but she was unhappy with the armhole finishing. The armholes were too snug because the ribbing pulled them in.
When I took out the ribbing, I saw the problem.
But first, some context: One really neat trick knitters love is a way to make the row-ends look finished without any fuss. If you slip the first stitch of every row, the edge ends up looking neat and finished. Great tip. Works well.
Alternately, it works if you slip the last stitch -- but not the first stitch -- of each row (just don't do both).
Back to the matter at hand: Thing is, it's great if that edge is meant to be a finished edge, like on a scarf or a blanket. It's a bit of a problem if you plan to pick up stitches along that edge. Because what happens is that each edge stitch on the armholes actually covered two rows, so when she picked up stitches, she picked up much fewer stitches than she should have -- so the armhole was too small (there were about 50 stitches around).
My fix in this case is to pick up a stitch in 4 out of every 5 rows (a standard ratio), inserting the needle in the stitches before the edge stitch, and in each stitch at the underarm bit (I ended up with 78 stitches). She also didn't want an inch of ribbing, so I just bound off the picked-up stitches to finish. This solution means there's a little more bulk in the edge because I picked up a stitch and a half in from the edge, rather than one stitch in.
If you're knitting an edge where you want to pick up stitches later, don't slip the first/last stitch to make a pretty edge.
If you're going to pick up stitches along the edge, go ahead and stitch the first and last stitches so you have a good edge from which to pick up.