That's a good question. I'm not aware of any research on this, and my instinct says it won't matter a whole lot.
But, I'd guess that setting it in the middle maybe sends a subtle message to respondents that you're not trying to influence their response, and perhaps reduces a little bit of the distance their mouse has to travel.
I think the one exception would be where you have count data. I would definitely default to 0 if you're asking something like, "Of the past ten days, how many days did you..."