Mostly, I fight two hands high with the rear hand holding the phone, my front shoulder protecting the other side of the chin, and my left hand is in front windshield wiping straight punches. I do from time to time switch to a Philly shell for counterpunching. I will go to more of a peek a boo style when attacking. I am asking this b/c it seems most people just stick with one. The biggest weakness I can see in doing this is that I can get confused hands and hands out of position.
You might do it out of being fancy, and style has its points. But against a more worthy opponent, it may force you to do something entirely different, since under stress you will revert to a 'default style'. Nothing wrong with that, keeping in mind that Switching up the position of your hands also changes your offense, and all the angles that you get to throw from. Toggling the position of your hands without predictable rythm is good way to keep them guessing as long as it doesn't compromise yourself. Normally being good at one format is enough to win most fights.
i change it, it depends if i am in offense or defense or both and it depends on my opponent... only problem is, what Pugsley mentioned, when it gets rough i get in my default position (auto-pilot) which can be bad, i get stiff, slow and very stressed. if i am much better then my opponent i get relaxed and then i think i can win anybody, i change guards, styles, try new things and im having control. So, i think its ok to be all-arounder but also to have control when it gets rough. i think andre ward is very good in that, he can go forward, backward, clinch and change styles..
I'd say most people will naturally alter their defense according mainly to two things: how they're feeling themselves, and of course their opponent. Of course as you all stated everyone has their most comfortable or "default", but again.. I'd say most fighters with some experience probably alters his guard and style in general throughout the rounds whether it be shadow boxing, hitting the bag, sparring, or a fight itself. For an inexperienced fighter, I'd say stick to keeping yours hands up to at least cheek level.