more scrittles

Yo, avoid this entire concept. I first ran across this in music school, where nearly every teacher would use this idea in one way or another. Basically, the thrust of it is that certain notes are supposed to sound bad with certain chords, so they should never be used. (One example is an eleventh over the dominant seventh chord, or a flat 9 over a half-diminished (minor 7 flat 5) chord. Personally, I think both of these sound great, especially the eleventh over the dominant - it removes some of the inherent dominant goofiness - it's become natural to me to always throw this into my dom chords now. And I can show plenty of tasteful examples of flat nines over half-dim chords, or even major seventh chords.)

The entire thing is repugnant to me, particularly as it applies to learning and practicing. Sure, it's a matter of personal taste as to whether one gravitates towards certain sounds or not, but for heaven's sake, those sounds should be as available to you as the ones you prefer. Maybe a flat nine sounds ugly to you in any context; well, if the situation calls for an ugly sound, isn't that where you'd want to be able to go?

Anything that limits the choices available to you should be thrown out. Ideally, an improvising player should always have an idea of what any note or chord will sound like before they play it, and that should be why they play that note or chord (as opposed to the regurgitation of rehearsed licks, which ain't improvising). So that player will always have the choice to skip or favor certain notes, as best fits the music they're currently a part of.