In my work, the best I can say is that I put fills or solos were appropriate. Because of the structure of most of my work, ie not necessarilly a traditional structure like AABA or whatever, I may use a fill or solo anywhere.

I use them all very sparingly though as if you listen to my stuff (at least as of late) I am taking more of a minimalist approach to the compositions. Meaning, less is better and what I don't play is as or more important than what I do play.
jam on,
Terry