I think it depends on the question, and, in particular, what the OP asks for.
- If a question is specifically asking for an answer with references to peer-reviewed sources, those references will be more reliable.
- Upvoted questions or answers on SE sites could certainly be used as sources, in some situations; you'll have to use your judgment.
- As freiheit pointed out, Sheldon Brown is a respected source for mechanical and general cycling info, and doubtless will be for quite some time. Other sites like Icebike, Rivendell, Peter White cycles, et cetera, are nearly on the same level.
- An article in a respected newspaper or magazine should be given more weight than one in, say, the Huffington Post.
- YouTube videos and suchlike content are, of course, the bottom of this ladder, along with pages on social networking sites.
- Look-at-this-neat-link sites (such as the ones in the Gawker network or Fark) are dependent on the quality of the links they reference. Some sites that serve up "articles" that are often just links are better thought-out, though, since they seem to spend time on sending you to quality articles. (For example, Slashdot or BoingBoing.)
Whether we need to worry about all this here is, again, determined by the question. If a "low-quality" source illustrates your point, and isn't essential to the quality of your answer... there's no reason not to use it.