I use mattress when I sew on sleeves. Instead of picking up bars on both sides, I pick up stitches from the sleeves and bars from the body.
I usually connect the sleeve to the body with a tied-on piece of yarn at the ends, the top and halfway down the sides. As you sew them on, you can adjust the number of bars/stitches/parts of stitches to ease the sleeve to fit. If you want, you can start at the top and work down each side, so if there is any unevenness, it's in the armpit.