I know I personally like to keep the opinion people have that I am overworked, whether I actually work hard or procrastinate - it's just a really convenient social shield, not only against failure but also against random requests from people. When people notice you have too much free time, for some reason they feel obliged to start inventing things for you to do.
I think it's an alternative to just saying "no". Some people are assertive and can simply refuse a request. Personally, I find assertiveness very cognitively taxing, not to mention socially, so maintaining a bubble of "he's constantly working on something" impression helps. It's probably not the most ethical approach, but it's a coping strategy.
As for self-deprecation of your work, I know I've been guilty of this at times to cover for stress and lack of motivation. It would be true that the particular piece of work is not the best thing I can do (and I can always point to examples of a better work), but fortunately people immediately assume (without me having to suggest anything) that's because of being tired, or new, or something, and not because I am having trouble finding the motivation to pursue a task at 100% of effectiveness. This buys me necessary time to sort my motivations out and hopefully regain the energy to do a better work next time.