What are we expected to do when citing literature?
There are (in rough approximation) three reasons for citations:

1. Giving credit

If our paper builds on other papers, if we have taken any ideas from anywhere else, if anyone did something we are doing before, etc, we have to cite them. Questions such as "Is this peer-reviewed?" or "Is this accessible?" do not matter in this category.

2. For proof/evidence

Often we will claim that something is true without establishing its truth within our paper. Then we need to refer to other work for this. 

Here, selectitivity makes sense: Citing someones blogpost claiming that X causes Y is not going to me particularly impressive. Citing a mathematical proof written in English available online in a journal is better than referring to a thesis written in an obscure language only availbale in hardcopy at some particular university (although do not forget 1.: if the thesis is prior, we may have to cite both). In sciences, it makes sense to some extent to cite multiple independent sources, less so in mathematics.

3. Providing context

We also cite to provide the broader context our research fits into. Some citations may have a "If you liked this paper, you may also like the following"-flavour. Here we have more or less full freedom what we want to cite. Pointing to one well-written survey article may be much better than citing 25 individual paper without much comment.