Because I tend to be lazy and want to have a finished garment, I pleated the two inches of excess fabric out of the sleeve caps. I wore it for over a year, then decided I didn't like how it looked. It sticks out and makes pointy shoulders. Plus, it reminds me of unflattering pioneer dresses in TV westerns. So I ripped out the seam and gathered the excess. During that year, I read Tasia's method of gathering and now I no longer dread this sewing technique. Before, no matter what I did, the threads would break or pull out or the gathers would slip while I sewed and I would spend hours trying to gather a few inches. Hence, my preference for pleating. With Tasia's technique, notably, securing one end of the threads onto pins before gathering, things are much easier.
Although every pattern I have seen says to gather the excess only between two marked spots on the sleeve cap, I don't like how all those gathers make the sleeve puffy and girlish. So for this re-do of a wearable muslin, I gathered from underarm seam to underarm seam, leaving only enough room for the underarm seam allowance to not be gathered. While this method produces some gathers in unexpected places, it also allows for a smoother, flatter sleeve cap. I have since used this technique on muslin sleeves that had less than one inch of excess and the sleeve cap came out completely smooth.
So now I've slightly altered my shirt and can get another year's wear out of it. I wonder what alterations I'll make next year?