iPhone Exchange account data use

I just discovered I can set my iPhone Exchange mail days to sync to "no limit". Good, but impact on data use concerns me. My incoming email gets promptly moved from my Inbox into a proper sub-folder under Inbox. At what point does data start being used - when I go into one of the sub-folders? And what if I go back to the folder another time? Does it just download the messages dropped in the folder since the first time I downloaded the messages or does it redo everything? It also seems that attachments (I get tons) do not download unless you tap the icon, which is good. In the end, I like the idea of having everything on the Exchange server at my fingertips, but accessing will be infrequent and I don't want to severely increase my data usage.