Andy, I meant gravity (on the ground) opened them, but as Steve pointed out, to prevent damage they would be 'closed' on the ground by the ground crew in normal circumstances. There are, of course, many photos of various 'marks' of '109s showing the slats open, partly open, and closed, as well as both, as in Steve's pic.
On those '109 kits where separate slats are provided, I tend to model them only just open a touch, as if they've 'crept' a little, just to show them, and to take advantage of the separate parts. (And no, I don't do that with the irritating separate flaps on Spit and Hurricane kits - they're modelled as they should be on the deck, if parked - UP !)