It's best not to use any symbol to group decimal sets of 3. Why?
In Europe 10³ + ½ is: 1.000,5
In USA one 10³ + ½ is: 1,000.5
SI standard for 10³ + ½ is: 1 000,5 (although the period (.) may be applied as standard for decimal separation since 1997 only in English being the comma (,) the standard separator for the rest of languages)
Way to confusing and useles to get concerned about. Anyway, we won't have anything bigger than a 10^4 number, so has very little relevance, if any at all.
FMI:
http://en.wikipedia.org/wiki/Decimal_point
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