If I were setting the counter against a wall with cabs above and the floor was full of whoops, sagging and uneven settling (often seen in older structures and or ones built during the winter) I'd still build the "real" kick into the carcass but step it back the thickness of the fuax kick to facilitate leveling then apply the faux kick to it.
In the case of the layout shown I don't believe it really matters much, there's nothing to guide the eye to an unleveled plane like a window or line of cabs. For me, if liquid flow is the only concern, (spilt drinks) it would depend on the extent of the un-level over the entire length of the island, including the el. Find the degree of end to end slope then raise the low end by half unless the low end is at the outside corner, if then, you also have the el to deal with and then I'd go for perfect level.
If so then you might end up running ply cleats/sleepers at the low end and then faux kicks make even more sense. It'll take some time but the island will still settle in due to the dead load of the footprint, (it's big and heavy) hopefully the joist cross the long end of the island, if not and you have access to the lower level you might consider reinforcing the floor by doubling up the joists and solid blocking in between.
Work smart not hard!
Never bite the hand that looks dirty