I have grass butting up to my deck, but mine isn't inline with the grass like yours. I do get some long bits of grass growing up the first 4 boards which I am able to clip. But if I was doing it again, I would put up a small rock barrier between the deck and the grass. Buy some 25mm cowra white (or stone of your choice), and run a 50-100mm strip of rocks lined with aluminium strip between the rocks and grass. That way you can cut the grass easily without damaging the deck with the whipper sniper and the grass doesn't get under the deck.