When I first learned to box I was taught to never backup in a straight line as this leads to openings around the sides for hooks and gives the opponent momentum moving toward you. Not a good thing at all!
In some "situations" though, I've found that a well timed and protected retreat can be very useful in changing an attackers timing and distance, causing them to over extend. This is a good thing!
Usually just a half step back, then move in another direction. I am just now becoming aware of the many advantages in moving within the circle. Very cool concept....... If you really want to see a true technician though, either catch a re-run of, or watch the next Roy Jones Jr. fight. He really understands the concept of body movment and placement. Best demo of Aiki outside of Aikido I've ever seen.
All I really know is I'll move any direction necessary to avoid getting clocked!
Dan P. - Mongo