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Testing is crucial to making a good level.
You don't know when something's too hard, or too easy, or simply not working. And even if you check for the former two, the latter is also super important to check. Glitches can happen any time, so, as soon as you change a script or a room in a major way, test. Test. Test.
Misalignments are usually easy to catch, so you only need to test the alignments when you're not sure about it. Still though, testing for misalignments is still a good idea. Meanwhile, again, glitches can happen at any time, so test every layout at least once.
Testing is one of the most important parts of building a good level.