I can only speak about British guns but in the 20 years or so I have been working on them, I have never come across a pre-1920 British hammerless shotgun with rebounding hammers (plenty of hammerguns but no hammerless).
I too do not 'slam' my guns closed but I see plenty of people on the clay ground and pheasant shoot who do and it only takes one....!
Could slamming a gun closed with the tumblers down fire a primer? It would depend on the degree of 'over-centre' effect of mainspring on tumbler, the mainspring's mechanical advantage, the strength of the mainspring and the softness of the primer. In truth I don't know but have no intention of finding out the scary way!
Bearing all the above in mind, I would suggest (IMHO) that having your gun cock before the cartridges clear the breech face is a wise move.
