I stand my playfields up. Less chance of someone or something stepping or dropping on them and breaking them. With the playfields packed away in boxes like yours Peter I don't think it would matter either way. If laid down flat, you would probably want to make sure they are fully supported in the middle of the playfield so they can't bow over time.
As Adam said, I think away from temperature extremes (heat, humidity and moisture) is the main thing.
