One thing not mentioned here is that exact matches will have a higher quality score than broad matches, so don't just rely on broad match keywords - include some exact match keywords for common variations.
QS as reported in the interface is a bit like tool bar page rank; it can tell you if you're doing really well or really badly but at the end of the day your competitors probably have a load of 7's too
Discussion about probable reasons why in the comments of http://www.rimmkaufman.com/rkgblog/2010/09/28/negative-keywo...
Because of some negative keyword trickery on the broad match version we decided it was a reflection of the account level QS