I guess it's all down to what kind of game you want to make.
If you're planning to churn out a lot of levels then a good editor is important, but if you're just using a few levels to show your ideas then it might be better not to bother.
In ROM CHECK FAIL I used a single text file for all my level data. It looks a bit like a nethack screenshot

Pro: Can use standard text editors like notepad, dead easy to import into engine, cost almost no development time
Con: Not much fun to edit
For Polychromatic Funk Monkey I used a combination of hardcoding in the game source (for the starting zone) and a variety of automatic level generation processes.
Pro: Zero implementation time, some replayability (which is okay for players, but brilliant for keeping playtesting interesting)
Con: Again, not much fun to edit. Also the automatically generated area could occasionally be unsolvable or un-fun, and eventually feels samey.