"My dishwasher has a little container for detergent which has a cover that opens to let it out. It can open but never close by itself. I load detergent the first thing before anything else, so if I open the washer and the cover is closed, it means it's still dirty. Indeed, you still need to open it but you don't have to scrutinize the actual load for cleanliness."
"If I had a reversible sticker, I'm guaranteed to forget to flip it when needed. The detergent cover is a better indicator for me. Granted, I only have to look at it a couple of times a year, mostly when someone else loaded it for me, which is rare."
I'm with you, I would be sure to forget the magnet, so I fill the powder dispenser the first time I put a dirty dish into the dishwasher.