working of round()

[Krishna.K.1900 at gmail.com]

Does round() always perfectly return the output expected or are there
some artifacts which don't allow perfect functionality

Using python 2.5:
>>> round(12.234, 2)
12.23
>>> round(12.234, 3)
12.234
>>> round(12.234, 1)
12.199999999999999
>>>

but was expecting 12.2

Also, for round(x,n), can't 'x' be an expression

round(5.25/2, 2)

was expecting 2.62  , but

>>> round(5.25/2, 2)
2.6299999999999999