What I have found is... it depends on how you jump!
A break tip should be relatively flat. I use about a quarter radius... being that I am trying to hit as close to centerball as possible.
However, for a jump cue, it depends on the method in which you jump. If you just try to get over a ball, and get a good hit.. then flat is great. However, if you jump a ball, plan on making it, and THEN get shape, a more round tip allows you to spin the cue ball when you jump.
My jump cue has the same radius as my playing shaft. I can jump a full ball and still draw 3/4 of a table... you simply cannot do that with a flat tip.