If you drop a night at the end of your stay, pretty much the worst thing that can happen is that the hotel charges you an early checkout fee of maybe $50 if that's their policy. However, telling the front desk when you check in usually results in no fee and no change to your room rate as typically the fee applies (for example in large convention hotels) if you tell them after check in that you'll be leaving early or don't tell them at all until you go to the front desk to check out on your actual day of departure.

It's harder to drop days at the beginning of a reservation without affecting the rate since if you no show on the first day, you lose the entire booking and are charged for the first night at most hotels.