It depends on who you're writing for, basically drosophila, so you're right that in 'everyday' writing (e.g. writing a letter, sending an email, whatever...) it's really up to you to decide which you prefer. (And, I suppose that if I were being really pedantic - I'm so good at it
- I'd say just be consistent, and don't mix single and double for no good reason.)
If you are writing for a publisher, though, you are likely to be told which to use. In the UK the 'default' is normally single inverted commas, and so you'd only use double inverted commas for a quotation within a quotation... as in:
Joe Bloggs writes that 'those women who say "snurk" are clearly Mumsnetters'.
In the US though, the 'default' is double inverted commas, with single being used for quotes within quotes.
I have seen double inverted commas used to denote speech, with single inverted commas being used for, say, quotations from a written text, but it seems wrong to me. I stand by my rule that you choose one and stick to it.
HTH