I would say it depends on whether arms are a strong or weak point. If you're arms are already one of your stronger points, you can save your energy for other stuff, and the arms will still be worked by other exercises anyway.
If they are a weak point they need to be trained or else everything else will keep getting bigger, making the arms appear even smaller, which is what happened to me. I figured oh my arms get trained with other movement, which the did but they didn't get big enough in proportion with the rest, so now they look small.