I don't believe in boycotting or backing any company based on past political contributions. It will more than likely be reversed by the next election cycle, so the one you choose to buy from today that backed Bush and the GOP in '04 could be backing Hellary and the DNC in '08. It all depends on who can close the deal at the last meeting before the check is cut.
VERY good points in your post. I remember seeing some of those "red" companies on conservative boycott lists in the past - Geico, for example. This is all a matter of practicality, and you're right, the distribution of donations can change dramatically from one cycle to the next.
Sprint has been my cellular service provider ever since they started doing business in that market. I've never had a problem with their service. Well, a couple years ago someone told me that Sprint was a huge donor to dems who supported gun control policies, and that made me go do some research. What I found was that their campaign contributions were pretty evenly split, and this holds true for all kinds of companies on all levels.
So it's basically corporate CYA we're talking about here. The only reason I'm going to stop buying a product is if the company makes very obvious, repeated public statements about its political leanings. (Ben & Jerry's comes to mind.)