It depends on the type of scale you have, but with some types of scales, they are sensitive to the foot pads being on a hard object, as well as if you are standing with your feet in the precise spot where they are supposed to be because of the way that springs compress inside the scale to measure weight.
If you place this type of scale on carpet, the bottom center of the scale will "bottom-out" on the carpet, taking weight off the scale's foot pads (and thus the springs inside) and disturbing the accuracy.
A good digital scale shouldn't do this, but I would do what Cassie suggests - try both ways, and see if there is a difference. If there is, the type of scale you have requires the flat floor
Female, 30 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 5/19/14 = 179.6 (net: -35.6)
Current mini-goal: 175 lbs.
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130-140 lbs.
"You don't have to change your life today. You only need to change your day today."