Deadlines loom over us, taunting us, whispering sweet nothings into our ear: "You can do it," "Just a little more," "You're almost done!" I'd prefer to dig a hole six feet deep and bury it, but alas, deadlines are there for a reason, if for no other reason than to keep us motivated. That doesn't mean a deadline should interfere with the quality of a book, but keep in mind that it's not just your deadline.
If you're working with an editor (and you should be--just saying), then it's not only your schedule you have to consider. It's unlikely that your editor is sitting around twiddling his or her thumbs waiting for you to finish your manuscript. If you've worked with your editor before, then you'll have a pretty good idea of their workload. It's important to be respectful of their time. Give them a time frame for when you expect to complete the manuscript. If you run into delays, let them know that too, and give them a chance to fit in another client while you're finishing up.
What about your release date deadline? If you're with a traditional publisher, they're going to set that for you, but if you're self-published, be sure to give yourself, and your editor, plenty of time to give the book the attention it deserves. I've done it both ways--rushed the editing (thank goodness my editor is awesome under deadlines), and I've delayed a release date by a month because rushing a release date was not worth sacrificing quality editing. In future, I'll stick with not rushing.
Where it doesn't hurt to motivate yourself, and set a firm release date when you begin writing a book, be realistic with deadlines. Quality is always worth more than quantity. If that means a deadline rushes past because you're busy writing a novel your editor and readers will love, let it rush and enjoy the ride (granted, this is just an opinion).
Do you set deadlines for you or your editor? Do you always meet your deadlines?
Note: My editor does not edit the "Working With An Editor" posts, so I take full responsibility for any errors.