Interface Builder has a couple of settings in each XIB file that could be important to the correct operation of your application, or at least your expectations. Here's a screenshot with the important bits highlighted:
The Developer Documentation contains the standard definition.
I won't regurgitate what has already said, go read that stuff if your not familiar with it. The most important setting is the Deployment Target. If you have been developing for iPhone OS for a few years, your Deployment Target might be hard-coded (like mine where) to a specific release of iPhone OS instead of to Default - Matches your Base SDK from Xcode Settings. The documentation suggests leaving it to Default..., and it seems like the right idea, one less bit of busy work.
The Development Target for a single developer or small team seems like an unneeded setting. Everyone is going to upgrade at the same time to the latest version of Interface Builder. I would walk your XIBs every couple versions to update the setting to the latest. Doesn't seem like any reward for being diligent here.