I'm not sure we should be pointing this out explicitly. I think that 90% of the time we don't want people to think in pixels. Also, when saving, Figure.dpi is just a default anyway. I'd prefer this discussion happen in savefig.

I assume pixels is more or less what people are actually thinking in most of the time, because that's their target unit if they draw to screen or png. At least, that's my experience. - Also it's the unit GUI frameworks, websites (often), image programs etc use. Maybe Graphics designers and TeX people don't.

I think even the web is moving towards device independence.

It's just a note, and people are looking for this info:
https://stackoverflow.com/questions/29702424/how-to-get-matplotlib-figure-size