I really like online check-in and have frequently used it. My requests have usually been honored as long as they were made in advance of the check-in process. Typically, my requests are things like floor number or room location, etc. and are not requests that are not likely to be avaliable, such as a room upgrade. As such, these requests are consistent with the room category that I have reserved. Other requests that have worked well have been those associated with a multi-room booking and room locations in that circumstance.
I‘ve used online check-in with room requests at both WL and AKL. Usually, online check-in allows me to completely bypass the front desk, which is very nice. However, I’ve had no issues with going to the front desk on arrival if I wish to discuss the room once I arrive and prior to room assignment or room entry (after online check-in). They are very friendly and accessible. In the current COVID era, I don’t think that you would likely be able to enter your room then switch to another soley upon realizing that a request wasn’t honored or available (which is not what you are asking).
I think you should do what feels comfortable to you, however. If you’re not comfortable with the online check-in, the front desk will be there for you and remains a great option.
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