Should workshop papers be cited (computer science)
Don't cite bad quality work as evidence.  Judge the quality yourself.  Only cite things if you think they are of a sufficient quality to demonstrate the claim you want to reference.  Don't rely on peer review (whether in a journal or conference).  Lots of poor quality work gets into journals, too.

Work in progress and early results do not equal bad quality. Something that is still in progress, or hasn't been rigorously demonstrated yet, may still be worth reporting.  This doesn't make a bad quality paper, as long as the paper is honest about the interpretation of what is presented.  Falsely claiming that early results are definitive is bad, but "Our early results seem to show this, but we aren't sure yet" can be a perfectly good paper.  There is nothing wrong with citing such results, as long as you similarly are honest about what (and how much) they show.

In short, There is no reason for a blanket refusal to cite conference papers.  Such a practice might make you miss important evidence.  Do consider the source and the quality of the work, and try to cite the strongest source(s) when possible.  This will often (but not always) mean preferring a journal article.

I'm assuming you are thinking of cases where you are searching for evidence in a particular area and can choose what to cite. Of course you also have an obligation to cite anything your work relies on or uses, regardless of the source.