I always use sage on both by default, especially on slow (image)board(s). I don't use sage when I have something major to contribute to the thread or I intentionally want to bump it(almost never).
I regularly sage on the imageboards now, even though I once considered it to be quite offensive. Here, I do it all the time since I have very little to contribute, what with all the ENTERPRISE-CLASSanus haxing going on.
On the imageboards it is considered offensive because the imagetards interpret it as ima gunna sage this thread cuz i dun like it and the post limit will be reacht soona.
On /prog/ and every other sane textboard it is considered polite because you are saying It is not my intention to bump this thread for I realize that I do not add any significant on-topic information.