I often check-in bags at airport- or near-by train-stations hotels for the day (or even longer). I never mention that I am not a guest of the hotel.

I just tip the bellman normally (1$ a bag, 3-5 $ in total) when doing so.

At airport hotel locations (even as a non-staying guest, I can also profit from the free airport-hotel shuttle, often from faster getting a taxi (and without airport surcharge), and from better and faster rental-car rates (like at the Mariott SFO where Hertz doesn't add the airport and Rental-car-center-bus-surcharges ) and the rental-car just waits out-doors.