That's a hard question. There will be times later on in your work where your understanding of fundamental concepts (that go beyond the straightforward coursework) will help you with something. But generally in those instances, you will be able to do a little extra reading and studying to help you with those later problems. If you are doing a lot of extra work now in your school studies, and it is bringing down your test/homework/lab grades some, I'd say that it might be too much extra work for right now.
I think that the best guide in your situation (upper division, getting some really useful, practical EE knowledge that you can use daily in your EE work), is to use the labs and maybe some extra projects to guide how much extra you learn. I've said before many times here on the PF that doing labs and research and projects and extra projects outside of school really helps you to see what is important in real-world work, and to learn to "ask the right questions" of your instructors and of yourself. So I'd say that it's fine for you to spend a little extra time learning about something unusual that you saw in a lab, or learning how and why the inside of an RF spectrum analyzer is put together, or when you use a horn versus a parabolic dish, or how you weatherproof different types of antennas, or what the approaches are for dealing with multipath, or what the bounds of CDMA are, etc.
I've definitely spent a lot of time studying theoretical things, but I've also found over the years in industry that real devices are what matter, at least to EEs. So basing your learning on real devices and your work with them, is usually a good barometer of how deeply and how broadly you should approach your overall studies in a particular area.
I hope that makes sense. There's a lab with this course, no? Can you get the lab guide early and read ahead? That will help you to gauge what extra materials you may want to study ahead of time.