You can decrease in ribbing, but not invisibly. Keep a stitch as your center 'seam' and decrease on each side of it. The ribbing will look like it blends down into the seam 'line'.
You can do a ssk right before this 'seam stitch' and and k2tog after it and you'll get the decreases leaning toward center.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry