I take a contrary point of view, in at least one case. If you are benefiting substantially from code proffered by someone on SO, then you can verify that it works and public scrutiny via the up/down votes similarly subjects it to a form of lightly managed peer view. As a result, you are encouraged to cite the poster or the SO post for the source of code that you use.
There are cases where code is posted to SO that is quite a feat, and proper attribution is the professional thing to do.
As for opinions and whether or not SO is a "reliable source", that is immaterial in the case of code. For other SE sites, such as the math site, proofs may be given that are novel and not commonly known. For that, it is also reasonable to cite the author.
The tricky part is that successive edits and refinements may make attribution murky. I have no answer for that, yet.