Generally I would not think this would be a problem. For example, OrderId may be a key field and there should be no problem I can think of using that as a chart dimension. (note my use of the weasel word "generally" which means I'm not absolutely sure...)
The typical warning about key fields is to not use them as a counter field. See:
http://qlikviewnotes.blogspot.com/2010/01/best-way-to-count-keys-dont.html
Sometimes as data modelers we hide keyfields so they don't get used in the UI. But that's to allow us to optimize the keys in larger applications without worrying about usage in the UI.
-Rob
http://robwunderlich.com
