To be strictly correct, you should remember that a Sequence Diagram is a dynamic view of the application, so, you must show instances unless your class has static methods.
But if you follow Mark Cade�s example you will be fine. I havent gotten my results yet, but I followed his advices, as others ranchers have done.
Hi Cleuton / Santiago , ...Then do you think, while displaying the class name directly in sequence diagram, should i write a note about this deviation ?