I try not to have people with colds in my house, but almost all of my friends are considerate enough not to even try. But I also hate to go overboard about it - I won't kick someone out of my house if they come in and show signs of a cold. I will take extra precautions to clean up after they leave, however.
Living in such an overpopulated city and working full time at a large law firm basically means I will be exposed to colds. I take as many precautions as I can (wash hands often, always get my flu shot, never share drinks with anyone, etc.), but it's tough to avoid germs and viruses altogether. One thing that does help is being open with CF at work and with my friends - it makes them more cautious and considerate of my health. Also I try to "do unto others" in that when I get a cold I am careful to work from home as much as possible and warn others, as well as wash my hands every chance I get. Just b/c my friends aren't CFers doesn't mean they can't catch my cold.
I will say that since I've started going to the tx clinic for check-ups every few months I have become much more germaphobic. Part of it is in preparation for life after transplant and starting good habits now, but a lot of it is that I don't want to be responsible for bring a single germ into the tx waiting rooms or center. When you are around people who are seriously immuno-compromised it really makes you aware of your own actions in that regard!