score:7
Tipping is discretionary but indeed there are many cases when it is expected. For this type of service, which I have used many times, I always give a tip. This is one case where you are depending on them and it is nice to show appreciation.
Personally, I see this similarly to a coat check. Once I get stuff I left back, I pay a dollar or two per item to compensate for the service, even though it was offered without cost.