I think so. Here are some reasons why some people feel otherwise:
I think that, unless the work site is a monster corporation that's subject to the same rigid rules of "fairness" as the federal government (which is bad for the work and the customers), people should be flexible. Granted, we know that some friends are going to create this kind of problems. We also know that many friends will be aware of the problems and will work to avoid them, just as we will. So unless we know ahead of time that someone we consider a friend is going to be hard to work with, I think we should avoid prejudices and deal with situations as they arise.
Personally, I don't spend much time with friends if they're not people I work with. These are the people from whom a postcard once a year is adequate to sustain the good will. I save my energy for relationships that are actually accomplishing something.