Okay talented edit-meisters,

I've read 2+ hours of posts looking for tips to a clean render. Now I have a question:
When would I want to use "stretch video to fill output frame size" ?

I've read the help info and can't get my head around it. I do notice that if I have it checked when rendering to an AVI (best quality), I see horizontal lines (interlacing?) where the fastest action is.

thanks
warren