I do sometimes. When I'm sitting in meditation I find that the sensations on my skin, a particular visual focus or all of the sensations coming in together are better things to focus on for me. When I'm stressed out or anxious or become aware that I haven't been aware as well as when I light the shabbos candles my focus goes to my breath first.
i think breathing and meditation [on a basic level] are natural when you get in a peaceful state. personally i let my mind and focus do the work, the breathing follows, although i usually start by focusing on my breathing. the only problem i find is that concentration on breathing keeps one aware - which is what it is supposed to do of course, sometimes it is good to let slip away.

bit to earthly for me :p