Question about # of broad modified keywords
I know with broad modified keywords, you can choose to have certain words mandatorily included in your keyword -- but I'm wondering as to how many broad modified keywords are actually necessary. Let's take the example of "black shoes" as the core keyword... if my broad modifier is +black +shoes, I assume that would include "cheap black shoes", so should we add +cheap +black +shoes as a separate broad modified keyword or is +black +shoes enough?
Primarily, my question is -- should we make a broad modified keyword out of every keyword (long and short tail) in our keyword lists? Or, are short tail keywords broad modified with the core keywords enough?