You know, there's a lot of times where I catch myself saying "I'm going to be spending a few months on this track", and I keep telling myself to shut the hell up when I say that, because I'll always end up adding incredibly redundant things in my mixes, and I'll add redundant instruments as well.
So, personally deadlines help me, but they are very stressful.
The best thing with deadlines is to find the sweet spot. But, of course it's impossible.