I got some new wheels for my tandem today, Rolf Prima Vigors. The tall aero rims required a tall valve stem, and the only thing the LBS had is a 700x18-25 size. The tandem runs 700x28 tires. I installed the 18-25 tubes along with the new wheels and took a quick spin, they seem fine. Am I going to destroy my wheels, break my neck, get arrested, die, etc. if I run these smaller sized tubes? By the way, I am very impressed with the Rolfs, they even came with a test data sheet that shows the tension of each spoke and the runout on each rim. (Reminds me of the data sheets that come with high performance *****s). The butt dyno says they feel alot faster than the DT Hugi/Velocity Dyad 48 spoke wheels I was running before these. And here is an unavoidable gloat, due to a miscommunication during the purchase of the tandem, I got the wheels as a free swap for the stock wheels that came on the bike (CoMotion Speedster). Hats off to Suncoast Bicycles in Inverness, FL!!! There are still a few honest businessmen out there!
Here's a data point: I've run various tube sizes on my MTB following the "whatever was available to fix the flat" philosophy, sometimes waaay too thick for skinny hybrid tires, sometimes way too thin for knobbies. I've never had a big problem with doing so. I suppose you should always try to match the correct tube size for the tire/wheel, and I do so when possible. But short of that, my feeling is do what you have to do to keep riding.
Small tubes have to stretch more to fill the tires, so if there is a small defect in the rubber, a scratch or thin spot, there is a small risk that the rubber will eventually tear or perf at the defect and fail. This will be highly tube variant, and if you stick to 120
gram or heavier tubes, unlikely to be a problem.
I've been running ultralight (thus ultrathin) 65 gram tubes sized for 700x19-23 tire in 25 tires and never had any issues. Buying one size smaller tube (if available) was a cheap trick to get an even lighter tube back in my ancient days of racing.
Theoretically, the tubes only have to stretch a little extra to fill your tire. As someone noted, any weakness in the tube will be magnified. A big deal? Probably not. But I'm sure there's a reason tubes come in different sizes. I'm guessing that the day you pop that tube and scrape your new wheels up, you'll be wishing it wasn't because the tube was a bit small.