I vote bad Idea!

Automotive fix-a-flat is for tires, not tubes. I have never seen it seal a tube. Slime for tubes would be your best bet, but will still only work so well, and do make a mess. I have found that a good puncture resistant tire and regular tube or thorn resistant for added protection. I used to have flats every other day. Now that I run tires with a protective kevlar belt I have went 2k+ miles without a flat. I can deal with roadside repairs at that rate.