Raft questions

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.

I almost always use a raft usually 3 layers, except in one case. Using a raft helps compensate for a bed that is not perfectly level, the first layer of the raft will be of varying thickness due to an uneven bed. By the third layer it will be pretty much level so the part will start out nice and flat for the first layer of the part. Also the raft is to me a little easier to pry up from the bed without damaging the part (and the Kapton tape bed) because I don't have to gouge the edge of the part to get it off. The one time I don't use a raft is when I want a nice glass finish on one side, for example I print out text, and print with the front side down directly on Kapton tape bed. Front surface is as shiney as glass. Back side will not be quite as nice.