Yet, "there might be some people in my target audience who don't have 3.0 enabled cards" is not really an assumption, is it?
I find it slightly amusing that on the one hand you're trying not to assume something but yet on the other you're making a huge assumption, specifically: "there might be some people in my target audience who don't have 3.0 enabled cards". That's not intended as a personal criticism, but is intended to call it out as an assumption; I'd strongly advise that you research and profile your target audience before committing to anything based on that assumption.
I'd say it's actually very open-minded to consider that something might be true.
... But aside from that, I use as old a version as possible, test on more machines, different cards,
and then static link to GL and the extensions I use, to avoid referring to deprecated, later maybe even gone OpenGL functionality in an external binary.