I am surprised to learn the product doesn't already have this capability.  This is a show-stopper for financials.  Without this feature, I have to write my own control.

I work in the Document Management Industry specifically with the process of collecting user input.  When collecting numeric data, we need to collect specifically what the user types and to prevent the user from typing in more data than what the field is specified for.  With the NumericTextBox, we need the option to turn off the rounding feature and to limit the input when the specified number of decimal places is reached (while the focus is still in the specified field).

For instance, if the field is specified to have 2 decimal places, the user input would be truncated when trying to type beyond that 2nd decimal place.  So, a value of 2.34 would be allowed. If a user typed 2.344, the value in the field would be stopped at 2.34; and, if a user typed 2.345, the value would still reflect 2.34.  As well, if a user continued to type input past the second decimal place, the value of the field would still show 2.34.