Full disclosure: I have a sewing/tailoring/pattern drafting background. The education I received in garment construction means I may see things differently than some other sweater designers. It doesn't mean that I'm right or they are wrong, it just means different designers have different approaches that will create slightly different results in fit and appearance. In general, those individuals with slim upper arms, don't need sleeve cap ease as much as those with a fuller upper arm do.
Setting in sleeves is often viewed as one of the most challenging parts of seaming a garment.This is because the shape of the sleeve cap and the armhole are not an exact match. The height of the cap is normally shorter than the armhole depth but it's total measurement may be equal or greater in length to the armhole. For a more in depth explanation of sleeve cap and arm width relationships you can read my post here.
If you are working from a pattern it has predetermined that measurement relationship. Keep in mind that row gauge discrepancies will also have an impact here.
I design my garments with the extra length (usually about 1inch) and ease the sleeve cap into the armhole opening.
I seam the sleeves and the side seams of the garment first. I start to join the armhole and the sleeve cap by stitching at the underarm and I mattress stitch evenly across the sleeve and the armhole in the sections where the cast offs were worked. Each stitch is of the same length. Then I join the centre of the top of the sleeve cap at the shoulder seam. I use mattress stitch and seam evenly for a half inch on either side of the shoulder seam. Next I give the work a tug, aligning the two pieces and make note of how much longer the sleeve cap is. I use mattress stitch to seam from the armhole to the sleeve first taking a stitch on each side that is the same length. Then I begin to alternate making the stitches on the armhole side slightly smaller than those on the sleeve cap side. I make a guesstimate at the ratio of row to row numbers. If I run out of one side or the other before I'm finished with the sleeve cap I pull out the stitches and adjust the ratio between one and two row stitches on the armhole side. The goal is to ease the sleeve cap in without any gathering or bunching. Make note of the ratio for the other half of the sleeve and the second sleeve.
If the pattern you are working with was designed with no ease you will be able to seam without easing and all your stitches can be the same length.