Should we use "buffer" or "buffers", "file" or "files", ...?
EDIT: I realized that I included my opinion into the "question", sorry for that. I will instead answer my own question with my opinion.
For uncountable nouns, the singular has to be used.
For countable nouns, let's ask English Stack Exchange: 17 in favor of the plural, a net score of 1 for the singular. That's pretty much a consensus in favor of the plural.
There's one reason to prefer an arbitrary choice sometimes: for the sake of possible migrations, it's best to align with existing sites (Stack Overflow, Super User, Unix & Linux, etc.) when applicable.
In my opinion tags should be in singular form unless there is a really good reason to use the plural form. I would much prefer "buffer" over "buffers", "file" over "files" etc. Usually when a question applies to foobars it also applies to a single foobar.
Of course there are always exceptions to a rule: for instance, when tagging questions related to multiple modes then the plural form "multiple-modes" comes naturally and should be preferred.