How often should we expect 400 Bad Request?

Based on what you are seeing, if the same IPs logging the errors can later successfully access the site, then it is probably very safe to ignore the infrequent errors for the rational that you have presented.

Just to expand on MrTuttle's answer, the 400 errors I've investigated in the wild are typically someone trying (unsuccessfully) to exploit your webserver, typically because they haven't noticed that you're not running a vulnerable version, or they just don't care. A fair proportion of the 404 errors I get on my sites are in the same boat -- attempts to exploit vulnerable webapps I don't have installed (phpmyadmin, I'm glaring at you).

Kudos for actually reviewing your logs! I'd ignore the 400 errors. 99.999% of the time, they're client-side, and completely outside your purview; network flakeyness or a spyware-ridden browser, in my experience.