In most non-endurance events, tire wear and fuel consumption aren't a factor. When you get to the longer races, your tires will definitely wear out.
I ran the Indy 500 the other day, which puts a heavy strain on the rear tires. You have to pit quite a bit, and the trip in and out of the pits takes a long time. If you use soft front tires and hard rear tires, you won't lose too much speed, but it'll keep you out of the pits much longer, somewhere around 25 laps. With soft front and rear tires, I was going into the pits every 10 or 12 laps, and the soft tires don't give you enough of an advantage on the track that it's worth pitting that much.