I use to work at a shop that EVERY vehicle that came into the shop got it's front windshield washed, and at least the front carpet vacuumed and dash board wiped down. No matter how bad it was inside. The company called it "clean & vac service N/C" Anyhow a couple of times a year at least we'd get someone that would come back or call that would say something to this effect My car was only in for a flat tire, but it runs so much better, smoother and better power, I think you guys might have mistakenly done a tune up. One guy was absolutely INSISTANT that we'd tune it up on accident because he was getting so much better fuel economy.
I'll tell you what, for me I feel good about myself and my truck when it's kept up. I enough being inside it more when it's clean inside, even if it's just the dash, windshield and hood.