I suppose I would fall into that category. I have some decent sized student loans, a marginal income, and little spending money. However, I also live in and manage a building with significant value in a nice neighborhood, and have the prospect of having my finances restored by inheritance, and a fancy degree and a realistic prospect of high professional achievements. I'd have to call myself upper middle class even without a ton of actual money on hand.
I've been told by my players that it's a point of swagger for them to be able to tell others they know someone who lives where I live. So I guess a small class difference is acknowledged, but not a problem. They're probably more middle of the middle; they have more money in the bank than I do, but somewhat less resources and status. They're definitely cool with having D&D sessions under my skylight.
Being inside the beltway as I am, I know many people who are much richer and more powerful than me, which usually seems to go fine as I steer clear of the nasty ones. Every now and then friends invite me out to something and my eyes get a little big at the cost, but by and large everyone gets along. Conversely, I work a lot with the military and took a pretty windy educational and professional road to get to where I am, so I've spent plenty of time with people who are-if not at the bottom of the social ladder-at least working class. That also goes fine. It's all roleplaying in a way; learning how to act in a way that doesn't send any negative messages to people who aren't like you. Class shouldn't really matter, after all.