IMHO || (a = 0) is way too similar to || (a == 0) and thus confusing. One day overzealous developer will just “fix it“, changing the meaning of your code. And every other developer will have to sit for a while to figure out whether this was your intent or just a simple bug.
And this is in fact what JSLint is trying to say:
Expected a conditional expression and instead saw an assignment.
I avoid using confusing constructs as they hurt readability. a = a || 0; is way more recognizable and similar in meaning.