The more precise your length and width are, the more precise your area measurement will be. Take this as example:
width = 9.3045 mm
length = 8.023 mm
Let's first round up:
width = 9.3 mm
length = 8 mm
Area = 9.3*8 = 74.4 mm
But if you are more precise with the width and length:
width = 9.3045 mm
length = 8.023 mm
Area = 9.3045 * 8.023 = 74.6500035 mm
The more precise your length and width are, the more precise your area is. Also, as you can see the more precise your length and width are, the more your area will increase and will be more accurate.