In my view, online check-in is only beneficial if you know the hotel well. Whenever I'm a first-time guest at a hotel, I always want to talk to a Front Desk representative because she or he knows the hotel better than I do when it comes to room assignments.

When you go online, you'll be assigned a room. There's no way to know if that room was assigned by the property management system or by a human who noted your preferences and selected that room because it matches those preferences.

You should be able to change your room, but unless you know the hotel well it's tough to know the best room. Online floorplans don't show locations of loading docks, garbage dumpsters, HVAC equipment, restaurant exhaust hoods, nearby structures, or some service areas. Unless you get a brand new Front Desk representative, that person will know the quirks of the building. She or he should be able to assign a room that is "better" than an auto-assigned one or one from on a basic floorplan.