Whiteboards are better than napkins for team-based visual thinking.
However, most large whiteboards are shared by multiple people and teams and usually hang in conference rooms. As with any shared space or object, cultural standards, civility, and politeness are required to maximize its benefit for everyone.
Scott Ambler has some good recommendations for whiteboards and whiteboard modeling on his AgileModeling.com site.This is a software development site - visual thinking, communicating and modeling are important parts of designing software-based systems - but these principles apply to any whiteboard use.
You can also download a one page PDF summary of his Whiteboard Etiquette Rules there.