You should always try to calculate your bracing. If you ignore this, then why not also ignore the space the speaker takes up (prolly approx the same)? After you start ignoring enough things, audible problems will occur.
If you know enough to do the math calc for your box volume, its the same calcs to figure any bracing needed (what, like 6th grade geometry here guys //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif). There's no real good reason to ignore these calcs (or to recommend doing so) imo, unless its just too much work for ya. //content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif