Or maybe, raft question. Is there a particular situation where one should use a raft?

Yesterday I printed a large-ish piece of a box; it's about 9x5 cm at the opening and rises about 7cm to a point. I thought this might be a good time to use a raft because I wanted to be sure the edges of the opening don't warp.

The raft ended up being 30% of the print time, though, which makes me seriously reconsider whether it's necessary. And I really don't know when rafts are necessary, hence the post. Incidentally, my raft settings were 3mm layers, 3mm offset, .14mm separation, and 20% infill.