DO IT! If it's at all possible, I always knit sleeves from the top down. You have no doubt about whether they'll fit in the armhole, you can make them the right length (you can try it on) and best of all, you decrease all the way down. I hate increasing sleeves. :o
Depending on how many stitches I start out with, I usually decrease one stitch on each side of the center underarm every 4-5 rows.
When you pick up the stitches around, try to knit into a full stitch (both sides of the stitch) to get a firm seam. I'm so happy I saw this post. It gives me a chance to spread joy!