pythonic use of properties?

[Marcus Goldfish]

I'd like advice/opinions on when it is appropriate to do
attribute/property validation in python.  I'm coming from a C#/Java
background, where of course tons of "wasted" code is devoted to
property validation.  Here is a toy example illustrating my question:

#   Example: mixing instance attributes with properties.  Is it pythonic to
#   validate property data in setters?  Since tens and ones are never
#   validated, the class can be "broken" by setting these directly
class SillyDecimal(object):
   """A silly class to represent an integer from 0 - 99."""
   def __init__(self, arg=17):
       if isinstance(arg, tuple):
           self.tens = arg[0]
           self.ones = arg[1]
       else:
           self.number = arg

   def getNumber(self):
       return self.tens*10 + self.ones
   def setNumber(self, value):
       if value < 0 or value > 99:
           raise ArgumentException("Must in [0, 99]")
       self.tens = value // 10
       self.ones = value % 10
   number = property(getNumber, setNumber, None, "Complete number, [0-99]")

x = SillyDecimal()
x.number, x.tens, x.ones        # returns (17, 7, 1)

Even though "tens", "ones" and "number" all appear as attributes, only
"number" has its input validated.  Since the class is designed to only
hold numbers 0 - 99, one can 'break' it by setting self.tens=11, for
example.  Should tens and ones be made into full-fledged properties
and validated?  Should number have no validation?  Is it more pythonic
to encapsulate tightly, or rely on "responsible use."

Marcus