Bitmap fill only uses the one image, so it doesn't add to the file size, -however- if you're gonna be creating a fullscreen flash site, complicated bitmap fills do tend to slow down the animation (tweens) a little. At least, that's my opinion in the matter.
General notice: from now on, I would like to ask everyone to put [AS2] or [AS3] (corresponding to the version of actionscript you are using on your project) in front of their post titles when the question is actionscript related! Please help us help you faster. Thank you.