" why do you think we round up when a 5 is the next digit to the right of the place in witch we are rounding ? "
i'm guessing it has something to do with numbers starting out at zero ?
Do you think it would be reasonable to give everytime all the numbers in pi for example?
One other thing... when you take a reading, you always take it to the nearest reading you can measure. If the exact length of something was 1.0652 cm, you will only be able to measure a length of 1 cm with a common ruler.
Rounding off is a means to simplify numbers and remove unnecessary details. This eases work. Where some accuracy is required, then you don't round off.
Here;s my justification.
When rounding a number, the critical digit can be any of ten digits:
. . . $\displaystyle \{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9\}$
It seems "fair" that half of them round-down and the other half round-up.
. . . $\displaystyle \underbrace{0,\:1,\:2,\:3,\:4,}_{\text{round down}}\:\underbrace{5,\:6,\:7,\:8,\:9}_{\text{roun d up}} $