I found something out with the RadNumericTextBox control: for the GroupSeparator, it will accept an empty string, but not for the DecimalSeparator property.  This is important, as the RadNumericTextBox behaves in a certain way.  If you try to set:

<tel:RadNumericTextBox ..>
    <NumberFormat DecimalDigits=”0″ DecimalSeparator=”” />
</tel:RadNumericTextBox>

An exception is thrown for this, but

<tel:RadNumericTextBox ..>
    <NumberFormat DecimalDigits=”0″ GroupSeparator=”” />
</tel:RadNumericTextBox>

will not throw an exception.

Let’s say you are using this control for the zip code.  The side effect of not specifying an empty group separator is that it will insert a comma every three places, and create a number like 90,210, whereas specifying an empty string will yield 90210.  Also, setting DecimalDigits to zero prevents using the decimal anyway.

For amounts, set the DecimalDigits to 2, and the DecimalSeparator defaults to period (at least for en-US culture), and comma (for en-US culture) every three spaces.