simple bitmap question

[PantherRun]

Hey out there;

I use many bitmaps in my work. Often, I'll have a bitmap that needs to have nothing done to it - say a background - and will be used only once on a movie. So i tend to leave it as a bitmap and not create a symbol out of it. Is that wise? Should all bitmaps, used on the stage, be changed into some sort of symbol?
And if so why?

Thanks there