When I saw "buttons" in the title I thought you meant on clothing. Anyways, I'm the opposite in that I refuse to touch any community or public button (such as in an elevator) with my fingertip/finger pad, and instead, use the knuckle of my index finger. Any button in public that requires pressing, it's done with my knuckle. The reason? Because they are filthy germy, and if I use my fingertip, this feels more contaminated than if I use a knuckle, because I never touch MYSELF with my knuckle, or handle things that I use with my knuckle; it's with my fingertip.
The buttons on a keypad at a grocery store checkout line where you enter your phone number to get a discount, I use my knuckle or the "pen" that's provided. I also use my knuckle for flat "buttons" that are on point-of-purchase screens like when it asks if you want a paper receipt, or if you approve the purchase amount or want to donate. Or the self checkout screens, always I use the knuckle.
As for turning public doorknobs, I don't have a problem with this, though I rarely see a public door KNOB. It's usually a handle of some sort.
Oh, the "button" to press at a gas station to start the gas pump -- I use either my car key or my foot. Do I want a receipt? I use my car key on the "no" button.
I don't know if all of this is due to autism???