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Sorry: that was nonsense. trunc is fine---it's round, floor and ceil that fail on large arguments with this patch:
>>> import decimal, math
>>> math.floor(decimal.Decimal("1e30"))
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/Users/dickinsm/python_source/py3k/Lib/decimal.py", line 1475, in __floor__
 return trunc(self.quantize(Decimal(1), rounding=ROUND_FLOOR))
 File "/Users/dickinsm/python_source/py3k/Lib/decimal.py", line 2265, in quantize
 'quantize result has too many digits for current context')
 File "/Users/dickinsm/python_source/py3k/Lib/decimal.py", line 3546, in _raise_error
 raise error(explanation)
decimal.InvalidOperation: quantize result has too many digits for current context
>>> round(decimal.Decimal("1e30"))
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/Users/dickinsm/python_source/py3k/Lib/decimal.py", line 1487, in __round__
 return trunc(self.quantize(Decimal(1), rounding=ROUND_HALF_EVEN))
 File "/Users/dickinsm/python_source/py3k/Lib/decimal.py", line 2265, in quantize
 'quantize result has too many digits for current context')
 File "/Users/dickinsm/python_source/py3k/Lib/decimal.py", line 3546, in _raise_error
 raise error(explanation)
decimal.InvalidOperation: quantize result has too many digits for current context
>>> 
Can I suggest using _rescale instead of quantize? For example,
 def __round__(self, ndigits:_numbers.Integral=None):
 """Rounds self to ndigits decimal places, defaulting to 0.
 If ndigits is omitted or None, returns an int, otherwise a
 Decimal. Rounds half toward even."""
 if ndigits is None:
 return trunc(self._rescale(0, rounding=ROUND_HALF_EVEN))
 return self._rescale(-ndigits, rounding=ROUND_HALF_EVEN)