I took advantage of a tight castoff to help make the collar roll properly on the cardigan pictured above. That's not the only technique I used, but it does make a difference.
So your cast off is too tight when you don't want it to be?
First, a little analysis as to why. Are you using an inelastic yarn? Is it your technique? Are you an especially tight knitter? Do you tension your yarn around several fingers? If you are really tight you can try loosening on purpose by not putting any tension on the yarn and unwrapping it from around more than one finger.
Recognize that casting off moves the stitches so they lay sideways across the top of the knitting. Knit stitches are wider than they are tall. Stitch gauge and row gauge aren't the same for this reason. The ratio varies depending on the knitter, the yarn and the stitch pattern used.
The simplest solution is to cast off with a larger needle. The actual size can be determined either on your swatch or your project as long as you are willing to tear back if it's not working. I usually start 2 needle sizes up using metric sizing in full millimeters. I like this method because my stitches stay very even.
If you think it's too loose, it's a really good idea to at least steam block the edge before moving to a smaller needle.
And for another post on the dreaded band flip with more blocking tips, go here.