I carefully air out my sleeping bag in the sun after I use it and it is still fairly new. I don't feel like it needs washing, but around the area (10"x3"?) where my neck and chin make contact I see just a little grime that has collected.
I was wondering if anybody had a recommendation for spot cleaning a small area like that. It is on the inside, not the pertex outside fabric. I was thinking of a dilute hand soap on a washcloth, but I would appreciate any suggestions.

Down doesn’t like detergent soaps but with a damp cloth and a drop of Nik wax down wash your idea sound sound to me. Rinse it good right there and let it dry. You might be able to move most of the down away and it should go fast. Woolite works well too but I can’t stand the smell of that stuff. Nik wax down wash doesn’t have an odor for me.

Arizona's post seemed sound to me. Before you decide to not wash your bag, consider whether or not the bag seems to be fully lofting. I have found that down items might not appear to be visibly dirty, but for whatever the reason, washing restores much of the fluff/loft. I generally use Atkins Sport Wash with great results.

+1 on putting the bag in the dryer, with some tennis balls or wool dryer balls.
We use the Costco down throw blankets at home a lot and periodically throw them into the dryer on the delicate or steam cycle which freshens and fluffs them up.

We have washed the Costco blankets on delicate a couple of times, but I'll admit I'm leery of doing that with my precious down quilts.

As mentioned on another thread, I washed my Western Mountaineering sleeping bag following the directions and products (ReviveX) stated on the Western Mountaineering website. It took quite a while, but the result was worth it. The bag fluffed up thicker than it was when I bought it, so it was actually a bit warmer!
Here is the information.