What do you think about creating pixel perfect wireframes? Good or evil? (Or it depends ;-)
The websites our team work on have a standard width, grid, interactive elements and some general guidelines for typography. Therefore any designer working on a wireframe could create it to the pixel (we have a fixed layout).
However, when I started out as a visual designer, I felt limited by the wireframes I was given in the brief if they were too perfect. They could limit the design, especially if they have been signed off by clients, who were now expecting the UI to look like a coloured in version of a wireframe. As I'm now responsible for interaction design, I'm careful not to over-spec, so there's more freedom in the interaction design/visual design collaboration. Problems can occur when developers or testers are working from an annotated wireframe that is visually different from the designs, sometimes triggering the wireframe-update-death-spiral.