If your boss gives you a project to work on, should you ever say "no?" Many people believe turning down an assignment from their boss would be career suicide, but there are worse things you can do. For example, you can take on a project you can't possibly complete either due to time constraints, for example you are already working on other things, or lack of the necessary skills. When you are confident you can't do an effective job, then it is better to turn down an assignment than to turn it in late or poorly done. However, discuss the problem with your boss. 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 offer you training.
While you shouldn't turn down an assignment on a whim, sometimes you are left with no other 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 might not be enough. Read Saying No to Your Boss.