In the past couple of months i have had to completely drain out gas tanks of a few classic cars that customers have been having issues with as well as rebuild carburetors and flush out the rest of the fuel system.
Don't know how the quality of the gasoline is in the rest of the country but here in AZ it is garbage.
I would recommend if your going to let your car sit for any length of time to drain the tank and run the carb out of fuel.

What do with all that gas you drain?

My '68 has been sitting for 5-6 years with the same tankful, with stabil in it. I started the engine a couple times the past month and it seems to run relatively well. I have been undecided on removing or driving the tank down. I can't use the gas in my mower or other vehicles.

Believe it or not if you put 5-10 outside in an open drain tub it will evaporate in a few days and a sludge will be left.
I have taken carb's apart that people have used stable, their was a brown jelly like substance in the fuel bowls.

Thanks. Don't think that would work here.

As for gasoline; I have to get 10% ethanol with 93 octane. The nearest ethanol-free (91 octane) is over an hour drive, one way. So, is it worth 1/3+ tank burned to get it?

I’d siphon the tank as low as possible. If gas looks decent you can always meter in a couple gallons at a time once on the road to use it up.

That may be what I'll do.

I've been using half a can of Seafoam for years out here in Cali. Haven't had any issues. Gas can sit as long as 6 months at a time, but I only keep 6-8 gallons in it and fill up if I'm driving any distance.

I have a 1968 Chevy C10 at my house i am doing some work on. Yesterday there was gasoline literately boiling in the tank and overflowing out the filler neck. That is just in the tank in a truck that sitting outside. You know what it does inside a carburetor that's sitting atop an engine where it is 160-200 degrees.
The gasoline today is crap