I was making a blouse and needed to enlarge the sleeve to accommodate my bicep circumference. The muslin I made of the altered pattern had draglines from the top of the sleeve downward. I checked my trusty 3-ring binder of tips and techniques and ran across this one that I'd photocopied. This is actually a tip from Sarah Veblen that appeared in the April/May 2005 issue of Threads magazine.
When enlarging a sleeve more than a small amount with the slash and spread method, the sleeve cap flattens. This can subsequently cause draglines from the top of the sleeve where there's not enough fabric. Sarah counsels that you should always check, even with patterns fresh out of the envelope, to make sure the sleeve cap is high enough. To determine this, measure with a tape your biceps's circumference by measuring around your arm at the high underarm and add the prescribed ease. Then, measure the distance from the tape to the shoulder point (that dent you feel when you raise your arm). The corresponding line on the sleeve pattern piece from the line along the bottom of the armscye to the top of the sleeve cap SA should be the same number of inches as the measurement of your arm. If it isn't, then the sleeve won't have enough fabric in that area and it is forced to stretch--hence the draglines. For more information on the subject of fitting sleeves, consult the above issue of Threads. This technique saved the appearance of the sleeve in my blouse by getting rid of the draglines.
ETA: Sara instructs you to add 5/8" to the measurement from the bottom of the armscye to the top of the sleeve for ease.