Is it ever okay to turn down a project your boss assigns to you? Many people believe doing so could be detrimental to your career. Under certain circumstances, however, it can be more damaging to accept it. Taking on a project you can't possibly complete on time or correctly can do your career more harm. For example let's say you are already working on other assignments or you don't have the proper skills. When you are confident you can't do an effective job, then you are better off being honest with your boss than completing the project late or poorly done. If time is the issue he or she may be able to help you prioritize your assignments. If your lack of a particular skill is the problem, your boss may be able to assist you with getting the necessary training.
You shouldn't turn down an assignment on a whim, but do so when you don't have another option. Find out what your boss should consider acceptable reasons for saying no to an assignment, and which ones will just seem like poor excuses. Notice I said should. When it comes to some bosses, a reasonable excuse may not be enough. Read Saying No to Your Boss.