First, I'm not convinced that a mix always needs to be left/right "balanced" such that you must have "two percussion elements so that one can be panned right and the other left". Listen to Santana, or Prince. Van Halen records often have the guitar on one side. Bauhaus' Go Away White is a particular fave with just one guitar, hard panned; a startling idear on that record that I love.
That said "balance" is often achieved by other instruments in the mix, guitar vs. bass, or bass vs. floor tom, etc. But there are numerous records with tambo all over one side of the mix only.
In this day and age, also, mix-effects are the norm, and so many are stereo, particularly reverbs but also delays and chorus, etc., that when something's up the middle (ex., lead guitar or vocal) it's spread by the effect; that can also be the case with hard-panned tracks, especially when (as is a neat and well-known trick) you hard-pan its effect (ex., delay on a guitar) to the opposite side.
I think of books like that one, and many others that I have read (I'm still reading Mixerman's first, haven't read the cited) as great sources of inspiration, and explanations of what has been done, that you might want to be inspired by, or re-create, but not, as crtjstr states, that you necessarily have to ...