Hi Chili!
Welcome from a fellow "newbie" in the Bay Area (CA)!
I think I would second my colleagues on the "galloping horse" test -- 3/8" in a double-sized quilt is a much different problem than 3/8" in a wallhanging, like what I like to do. I truly don't think it is a big deal.
I'd suggest that you just put your borders on as usual, but take an average of the top, middle, and bottom width (or length, if that was where the problem was) and cut your borders to that measurement. Pin the ends and the center of the borders/top together, then maybe 1-2 places equidistant between those points.
I will always remember the advice given to me years and years ago: if you need to ease in a bit of extra length, be sure to "sew with a loose bottom" -- meaning to put the fabric to be eased in on the bottom and the shorter fabric on top. Your feed dogs (the little "grabber things" under the presser foot that move the fabric along) will do the rest for you.
I think you'll find that your 3/8" will work itself in between your pins and you'll be fine...
And on the off chance it doesn't? Put that end under the pillows at the head of the bed and just enjoy the rest of the quilt without guilt!
8)