Most decreases lean a little, at least the stitch where the decrease is. There is a centered double decrease that stays straight if the leaning stitches are what bother you.
I think I would just throw in a row of decreases once in a while and write down where I did them so the other one can be made the same way. Decrease maybe 3 or 4 stitches evenly spaced around the arm. Knit a while and try it on and see how it is going and do another decrease row when you want to. Often decreases are done over each other so that they show, but if you want them more unnoticeable offset them halfway between the first decreases. Repeat decreasing as desired to keep the fit as you want it.