I would say it's a faux pas, but an easily forgiven one. I usually stick with standard rail passes, but I've bought Green Car rail passes a few times, and I have jumped on Shinkansen trains without reservation. Yes, the conductor will stop and talk to you if you sit in a seat that the tablet shows as unoccupied; upon showing the Green Car pass and telling them where you're going, they will either bow and let you stay, or (in case the seat you're in is reserved by someone else for part of the journey) usher you to a different seat. In case the entire carriage is booked, I suppose they might make you move to the non-reserved standard section, which might mean that you get stuck in a middle seat, or without a seat at all. So I suppose sound advice is to reserve ahead if you can, but it's not the end of the world if you don't.