sage has different implications on futaba-style and 2ch-style boards. On futaba-style boards where you have a low bump limit, sage is often used to show displeasure with the picture, since each sage will bring the picture closer to the bump limit. On a 2ch-style message board where the bump limit is much larger, or not existant at all, sage is used out of politeness, so as not to bump a thread if you don't think your reply is interesting enough to warrant the attention of everyone reading the board.