While I don't normally believe anything I see on Mythbusters, they tested 'double dipping' to see how many germs transfered into dips when you dip food that's already been in your mouth. It pretty conclusively showed that very little germs transfer into the food and that there's actually more germs in the food you're dipping into than will get transfered. I figure it's the same with tasting from a spoon.

I suppose I might use two spoons if my nose was streaming and I was cooking for anyone other than my other half. But the chances of cooking for anyone else is slim anyway, and I doubt I'd offer to cook if that ill!