As a boss I have some thoughts on this. It is okay most of the times to say no to the boss in a roundabout way. It is almost never okay to flat out say no to the boss. Frans solution below is also a good solution in some cases but done incorrectly could cost one their job. Often as the boss I want a certain person to do the job - not because I want to overload that person but because that person is the best one for the job that needs to be done. If your boss is passing work to you that means he trusts you the most to get the work done properly in other words he has a lot of faith in your abilities.
I once had a job working in a shop when i was younger and they would try to force us to work 6 and 7 days a week and the reason was because people in the office would make promises on orders and try and foce us to work like that i never worked 7 days a week and rarely 6 because i told them if the people in the office want to make promises and still get to spend the whole weekend with thier families it isnt right i only worked more than 5 days when the people who secdualed the orders would work the weekend too so basically i didnt come in weekends ever people should think about that when you have to pick up the slack for other peoples stupidity. i told the owner of the company this
and got promoted