Cary: it's not so much turning down a job, as going to look at one and then not pursuing it because you don't think its a good fit , or not even going to look at all.
I've dismissed many calls out of hand, either because it didn't sound like the project would ever get off the ground, or I didn't think the caller and I would see eye to eye on design issues, or because it was too far away and I wasn't comfortable sending my associates there (Russia, Africa, Sri Lanka).
I've gone to look at sites and then never pursued them because of the above reasons, or because I didn't find the land inspiring, or because I saw some problem with the land that I didn't think would let me build 18 good holes (not enough land, too steep, etc.).
I've declined potential projects in really good areas (like the Sand Hills) because I reasoned something better would come along later there, and I know I'm only likely to build one project in a given market.
I've even turned down jobs and then later reconsidered because the client kept pursuing until I was convinced they really wanted us (The Renaissance Club was one of those).
There are times when I wish I was busier and hadn't passed on something (which happened three years later than advertised and would have filled a hole for us, instead of stretching us thin). But in the end the projects one turns down may have as much to do with establishing a reputation for quality work, as the ones that one accepts.