General ⇒ why does my \parindent not work?

[localghost]

Yes - I realise this, but I'm not really comfortable about relying on blank lines to define my paragraphs. I'm going to have a document with a LOT of diagrams in it, so the presence or absence of a blank line in the file will not be so obvious. [...]

From my point of view some \par commands are less obvious than blank lines.

[...] On the other hand, it's easy to forget to include \par at the end of each paragraph. [...]

That's easier than to forget a blank line.

[...] Is there a general consensus on what's better? Or is this one of those "personal preference" things?

The latter one is correct (as you can assume from answers above).