I cannot speak for anyone but myself: I always measure from the wall to the sound source which is, generally, the front panel.
The front wall, although I do think speaker depth could be an issue if your speakers are box speakers that happen to be rear ported. For example, I had to put my WATT/Puppies further out into the room than my previous speakers (Tyler Acoustics) because the Tylers were front ported, and the Puppies are rear ported, and significantly deeper.
Here is some info from the Cardas website that gives a formula for speaker placement.
My conditions do not permit the distance from back wall so I cannot speak to the exact outcome.
Speaker placement, simply stated
The distance from the center of the woofer face to the side walls is:
Room Width times .276 (RW x .276)
The distance from the center of the woofer face to the wall behind the speaker is:
Room Width times .447 (RW x .447)
This is all you need to know to place speakers in a symmetrical, rectangular room!
Here is a link to the page (there is more information and a calculator too) http://www.cardas.com/content.php?area=insights&content_id=26&pagestring=Room+Setup