I don't own a 2004, so I don't know how many systems are affected by the wheel calibration procedure that bradca21 referred to, but you should always be careful about changing the wheel size on any car, because it could make the odometer and speedometer read incorrectly (7.5% low, in this case). A speedometer that reads low will make you more likely to get speeding tickets, and selling a car with an inaccurate odometer reading is also illegal.
The 2004 Prius stock wheels are P185/65R15, which means the sidewall height is 185 mm * 0.65 = 120.25 mm = 4.73", and the overall tire height (unloaded) is 4.73" + 15" + 4.73" = 24.47". This means the circumference is pi*diameter = 76.9". This means the 15" wheel will make 824 revolutions per mile, because 1 mi = 5280' = 63360".
A 17" radius wheel will have C = 83.15" (assuming the tire's sidewall height is the same) and will make only 762 revolutions in the same mile. If the odometer is calibrated for a 15" wheel, it will display 0.925 mi when the 17" wheel has actually pushed the car for 1.00 mi (7.5% low). (In other words, when you sell the car after driving it 100,000 mi, the odometer will only say 92,500 mi.)
Similarly, if you travel this mile in 1 minute, the speedometer will read 60 mph on the car with 15" wheels, but the speedometer on the car with 17" wheels will think the car is only traveling 0.925 mi / 1 min = 55.5 mph (7.5% low).
I recommend you perform the following test:
1. With the stock 15" wheels installed, drive alongside another stock car for 1 mile at 60 mph to make sure your odometers and speedometers read the same.
2. After installing the 17" wheels and performing the calibration procedure, repeat the comparison against the same stock car and make sure the odometers and speedometers still match.
Of course, if you can find tires with smaller sidewalls, such that the overall diameter remains the same, you won't have to recalibrate anything. For example, P185/50R17 (if they can be found) will have nearly the same overall unloaded diameter as P185/65R15 tires.
Douglas (2002 Silver, Wisconsin)