Usually I use fills at the end of a verse or a chorus, to introduce the following part of the song, but this is not a strict rule; sometimes, like in Big Band styles, a fill is useful even in the middle of a section (especially the chorus) to recreate the feeling a real Big Band has; other times (like in my rendition of Wave) I didn't use any fills at all, because the style I used (SD1 Soft Bossa) has fills that IMO are way too emphasized for that kind of music (the break fill even uses timbales!); besides, I didn't want to distract the listener from the music flow. To think that for that song I was (friendly) criticized because I left the fills out!
An important issue, to me, is the right timing to trigger a fill and that depends on the keyboard you are using. I have seen that the 9000 pro is quite tolerant, from this point of view, while with the SD1 I have to press the fill button slightly before the end of the bar to trigger the fill in the following bar. The VA7 is yet another story, but I cannot say how many beats before the end of the bar I have to press the fill button: only experience can make make you perfect.