Hilton has let you choose your room online the day before your arrival date. So far, it seems like the computer system only offers the worst rooms in the house, and even overlooks the room type you originally reserved. I've used the online check-in five times in the past 2 months, and every single time, the crappiest rooms were offered (small, next to elevator, ice machine, and so on). For whatever reason, Hilton can't seem to program their Room Selection Screen to offer rooms in the category for which you actually booked and paid. Twice, I had reserved a junior suite, and both times Hilton offered me standard rooms.
I use Hilton as an example only because I imagine the kiosks might also be limited in recognizing the room category originally booked, and then throwing out to you the worst rooms available. If you care, then I guess, you'll walk straight from the kiosk to the front desk queue.
I must say, I agree with speck72's comment above, that people waste a lot of time checking in. At the Cosmo, for example, it is typical to see folks spending more than 15 minutes checking in, and trying to score a Wraparound Suite for the price of a City Room.
I never understood the $20 trick. Why is it okay to ask the clerk to "cheat" the hotel by giving you a "free" upgrade for a tip that goes in the clerk's pocket and not into hotel coffers?