I alway go upside down burger too. It just seemed so intuitive, I never even considered it was upside down until one day I was confronted about it.
I always pick up the burger by scooping my fingers underneath to support the probably soggy underbun, with my thumbs on top. Then the burger just rotates 180deg towards your mouth. The top bun is always thicker and more sturdy, so it makes a great foundation.