I tried the $20 trick (usually don't for a 1 night stay) based on the horror stories read here regarding the "Classic" Rooms. I was pleasantly surprised the "legitimate" upgrade was $30 when if I originally booked the same room I would have paid $60 online with no guarantee regarding view. I probably would have been fine in the classic room (especially since it was only one night) but I really feel I got my money's worth with the excellent Augustus Tower room including the fountain shows.
My advice to you is to ask at check in. If many rooms are available they may upgrade you for the $20. Have a specific room (tower, view. etc.) in mind while asking. If they cannot upgrade you for the tip, ask what Harrah's will charge and you may be pleasantly surprised (upgrade at checkin cheaper than the same upgrade booked upfront). Many people bail after the $20 trick fails but I always ask what the additional cost is for my own knowledge and if it is reasonable and I really want the upgrade I'll take it.
I love the $20 trick for several reasons but the main reason is you cannot guarantee squat (view, room type, bed, floor, etc.) when booking your room.
If rooms were like tickets to a ballgame, you would know exactly what "seat" you were purchasing (with a seating chart). If you could click a specific room and pay more (or less) for the specific room you are looking for (or less if you want to save money and do not care about view or bed size or smoking / non-smoking), that would be better for the consumer and the casino could make additional money based on this scheme.
However, people who pay upfront an extra amount for their preference many times get a inferior room vs. someone who walks in and either asks and or asks and tips for an upgrade. Therefore, I rather know exactly what type of room I will get before paying more (either to the clerk and or hotel). I tipped the clerk for letting me check in early and her service on top of the upgrade charge from Harrah's. I rather have the clerk get extra money than the casino but that's just my viewpoint.