I sue it both planning and traveling.
In the planning stage, I look for where Street Atlas says I should look for fuel. Then I use the web GasBuddy to look at local prices. For example:
If you buy fuel in Ohio instead of Michigan, you can easily save 0.15/ and maybe as much as 0.20/gal.
If you buy fuel in Indiana, you can save 0.20/ to 0.25/gal over buying in Illinois.
In case you didn't do the math in your head, if you are only buy 50 gallons, 0.20/gallon is 10.00$us saved. That is your 10.00$ tax free. If you don't want that 10$, put it in an envelope and mail it to me.
Matt