These days, theme's are viewed much like contributed modules which code you usually do not alter but rather override in other places. Personally, I think themes are quite different from modules and that there's a good chance that future ameliorations of a theme produce unexpected results on your website but, nevertheless, this seems to be the current mainstream thinking nowadays.
Yes, that's exactly what I feel. I was always nervous that I will make a subtheme and then in 2 months we will upgrade and then my client will call me on the phone frantic that I "broke" page X. He won't know that it's due to a CSS change in the new version of the theme.
Has anyone else experienced this? Or is it basically considered safe to use a subtheme and rely on the main theme author not to do something stupid?
Yes of course one should do upgrades on a test site etc. but everyone knows that for a simple module upgrade, just making a backup of the DB and old code should be enough--if something goes wrong, the site can be restored in about 60 seconds. I would *tend* to think the same for themes...
F