I agree, no one will purposely run the tank dry. If you fill a bit at a time like you're describing, your lifetime mpg will certainly be ok.

What you'll miss is the ability to compare the mileageyou get out of different tanks of gas. Say 87 from Shell, vs 87 from Amoco/BP, or one with ethanol, etc.

I think it's more of an exercise in futility, but I'm certainly interested in trying it out!