Your profile reports your location as Portland, Oregon, so I'll assume that's where the dahlias we want to overwinter are located.
Let Dahlia tubers remind you of Goldilocks and the Three Bears--we don't want things "too" anything, we want things just right.
- Don't dig too soon, don't dig too late. Dig them up ten days or so after the first freeze or, if no freeze, in about mid November.
- Dry-ish, not so dry as to dry them out. Regardless of whether you use composting medium, sand or sawdust, the natural humidity of the area where they will be stored will effect their condition. If they start to dry out, you'll need to supplement humidity by using a spray bottle to dust a little moisture on the medium.
- Cool, but not too cold. A temperature between 40 and 50 degrees fahrenheit is "just right."
Overwintering your tubers is not a "set it and forget it" business. If you check the tubers every few weeks, you'll catch a problem before you lose the crop. The tubers will tell you if they are too dry, as they will start to shrivel.
Dahlias are beautiful, so the little extra care and keeping of the tubers is well worth the effort.