There is a specific rule(Banker's Rule I think) for rounding of numbers that end in 5. The rule is that we add 1 to the preceding digit of it's odd but keep it as it is if it's even. It's always keeping it even.
Why is this rule applied? I read something like it is done to prevent biased rounding off our something.
Wikipedia has a list of six different ways to round halves. To see the logic of round to even, consider adding a long column of numbers that are all an exact number of tenths-that all have exactly one decimal place. Now consider the effect of rounding to integers before adding. If the decimals are evenly distributed, the increases in the ones with $.6, .7, .8, .9$ will be balanced by decreases in the ones with $.1, .2, .3, .4$. If you round $.5$ up, as some have been taught to do, the sum will increase. If you round to even, the sum will increase half the time and decrease half the time.
