I found it to be a bit of a chicken-and-egg problem. At the time I learned it, I didn't really understand why relational theory was important. I trusted that it was, I just didn't really grok all of it. But later I found it of immense practical help when writing, and especially debugging, sql queries.

Perhaps the ideal order to learn in would be practical, theoretical, practical (applied theoretical), though that's just a gut feeling.