As a mid-continent American myself, I indeed use "Sir" quite liberally, including with some junior collegues (even on occasion with my own son). It is meerly a minor gesture of respect; an indication that I'm taking the other person seriously.
There are a few occasions when it gets me into trouble. In particular, with people who are active duty enlisted military. They are not entitled to that form of address in the military, and can become perversely proud of that fact. Using "Sir" with such a person can offend them.
Also, using the equivalent female form of address, "Ma'am", with someone in the vicinity of 30 can often produce a rather negative reaction. In that circumstances it implies to them that I consider them older, and some women can be fairly sensitive where their percieved age is concerned.
In general though, it almost never hurts to be respectful with other people.
I could see where this mode of address could lead to awkward situations in England, where some people are actually entitled to that form of address and some are not. We don't really have that issue in the USA though (outside of the military), so the title doesn't really have any connotations of class here.
My last sentence above is kind of an oversimplification (or a downright lie, depending on how you look at it). We do have class issues in the USA, but they tend to be racial. For this reason, while it is quite appropriate for me (a 40ish white male) to address address a black collegue with "sir" or "ma'am", probably both of us would very much prefer if the black collegue didn't respond in kind.
There's probably much more that could be said about this, but frankly this is about as far into my country's cultural pathology that I have the courage to publicly delve.