Question
I just upgraded to 1.9. Christian recommends against the \pgfplotsset{compat=newest}, but that leaves one having to come back and update compat=x.y in one's projects, several times, over a long period of time. I realise that if the project originally worked there's no need, but that's not always the case. Plenty of my projects get reused and improved. But, it's not a big deal if there really is no better alternative. What would be considered a valid use of compat=newest, if there is one. And, is it one thing or the other - compat=x.y or compat=newest?
Conclusion
The bulk of Christian's answer is about the utility of being able to set the maximum version on a per project basis and not only that, it highlights a built in feature that can actually recommend an appropriate version. None the less, the last paragraph is the telling one. It's better to have compat set to the latest version you currently know gives you what you want. It may seem that newest is a good idea if you're up to date with latest, but it won't always be.
So compat=newest is an ironic feature. It seems there is no Do. The Dos apply to the other settings of compact and the facilities they provide.
writelatexetc. though it is possible to figure out via\pgfplotsversionin the document. Or if I need to test a generic method with the latest version automatically – percusse Oct 21 '13 at 00:44compat=newestanyway? – Geoff Pointer Oct 21 '13 at 08:49compat=newestcan then be used during "development" of the document and frozen to a certain version once the document is ready to be prepared for final layout. – ThomasH Oct 24 '13 at 10:55