The previous chapter in BOMP was about breathing, which I equate to a large degree with relaxation. The current chapter in BOMP discusses the importance of proper posture. I have posted in the past on the idea of the relationship between posture and relaxation but I think today I'd like to grind this sacred cow into cheeseburger...
See, the ideal of posture in much of the Japanese martial arts world is basically SRU - Stay Rigidly Upright. I think SRU is a pretty good first approximation of a generally good, useful, efficient posture, but when you think about it, there's not too much you can do if you dedicate yourself blindly to the SRU ideal. In order to do anything, you have to move, and in order to move, you have to relax your hold on that SRU ideal.
Now, I do think that your postures that you use in the martial arts should largely revolve around a neutral, relaxed, upright posture (shizentai). But there are times where it is appropriate to flex or bend or even [gasp] lean over!
When you don't have some other better posture for what you want to be doing, you probably want to be in shizentai. But if it will accomodate your goals and strategies better to lean or bend or flex, then by all means, do.
Gentle, flexible, yielding-but-strong, flow in and through and around shizentai - that is my postural ideal.