A buffer is life saving !
I agree with @jcmraz on the fact that no buffer gives you a steady goal if you find it hard to motivate yourself. It works well with short stories/comics, when the ideas multipliate and you write a lot.
But in the long run, you're bound to have some impromptu in your schedule, may it be a simple cold, and it could lead to many hiatus, stress, and the worst : rushing.
You know, when you can't really plan ahead and bit by bit you just write what"s on your mind and if you're unlucky, it gets out of hand, into a spirale of inconstencies (yet you may be lucky and have everything that wraps up nicely)
For my part, it was so chaotic I had to go on hiatus for a year until I could get the story back on rails @__@.
So I would say, it greatly depends on the length of your story and how you feel secure.
No need to have a big buffer. If you shedule your updates at the rythme you write, even 2-3 chapters in advance may be sufficient to keep you motivated (when you have a really big buffer, slacking is really the danger, exactly like @Rhonder said ) and prevent any incident.
@Fuzzyman818 : that's quite a good idea.. Never thought of that kind of thing.