I learned this same trick from a post here by Raybaudi. As far as I can tell, there should be no trouble using this as a calculation to insert the comma as a thousands separator. But in many situations, such a calculation is unnecessary as you can use data formatting (even on merge text--both fields and variables) to add the same formatting.
Thanks phil. I need to display a calc with percentages, currencies, and text so NumtoJtext based calcs fit the bill. Good to hear its safe, thanks.