Tough, tough crowd.
I'm a last-minute man, always, and especially at an airport I know.
In Europe, though, they've different notions of time. A 4x/hour shuttle schedule wouldn't cut it at a "first tier" rental firm in the US, or at least not at any reasonably main-stream US airport from which I've ever rented a car . . . except maybe some group-bus-only places like Logan or BWI. So, as I said, knowing the airport then becomes especially important.
Oh, and if you're really going to play the last-minute game, yes, you actually do have to know the shuttle schedule in advance and calculate which one you have to catch. There's some expertise involved here, in other words.