Oh yes...A well-behaved horse rarely kicks out, but it's not worth the risk. I've been kicked once, in 18 years of riding, and it HURT like heck! My thigh bone was bruised, and I had bruising on my skin for 2 months! It looked terrible. I was helping with therapeutic riding, and the horse kicked out and got me while I was picking up items for a game. Needless to say, this horse, Tiger, did not stay in the program (it later charged the owner's son). So, anyway, I am wary of even the best behaved horses because once was enough. However, most of the time most horses will not kick you.
An important note: if you MUST walk behind a horse within "kicking-range," put your hand on him/her first and talk to the horse to let it know that you are there, so that it isn't startled. It will be less likely to kick you. Also, walk as close to the horse as possible. This will limit your chance of severe injury as the horse won't be able to kick you high (i.e.--your head), and the horse will not be able to get the force and speed of a hard kick with you close to it, as it won't be able to fully-extend its leg.
Again, though it's best to stay away as often as possible, and new and inexperienced riders are always told to never walk behind a horse, as they don't know which horses to watch out for, etc.
Also, in many shows, you may see horses with a red ribbon in the tail. that is a sign that this horse kicks...sign that it kicks other horses, in general, and to stay away. I just don't trust kickers in general--horse kicker could be a people kicker too....