I generally end up learning only a little more than I need to solve my clients problems and then extending what I know as I go along.
So before I started doing a lot of WP work (which is most of what I do lately), I learned how to setup a wp site. At that point I just customized other folks themes. Then I learned to do themeing myself, and found some more work doing PSD-WP conversion. Now that I've been doing a lot of themeing, I'm learning how to do more complex things like create plugins, and I forsee even more work doing that.
But it all comes down to how much of a need you foresee for this stuff. I do a lot of work for designers who can't or don't want to code a site, and they are happy and make money without having to learn this stuff.
But at the same time, I have okay design chops but find the money and volume of work for development a lot more attractive. Its all a very personal kind of decision. I am a lot more employable as a WP dev than as a designer, but that might just be a comment on my design chops