My own bias is to use the best sourcing available. If there is a more reliable source, may as well use it instead. If multiple sources disagree and could be considered reliable, may as well mention their disagreement.
Given the wide areas of the wiki with poor or no sourcing, it seems best to focus attention (when needed) on areas and details which are presently contested.
Yes, we should encourage sourcing. Yes, more reliable sources are better. If a topic is such that it is potentially contested and has no reliable sources at all, maintainability becomes an issue (this is one reason, I think, why we delete pages for reasons of notability: if nobody's written about a subject, we have no solid way to resolve disputes or even tell if some articles are accurate at all).
-Luna