Datajax:
The rules are ambiguous. The closest you could get to playing by the RAW would be to roll for each unit simultaneously with a different colored die and then move all your Reserve units on simultaneously with some sort of big apparatus to help you pucsh them all at the same time.
Since that is rather impractical I've seen the vast majority of people roll for all their Reserves first and then move them on the table.
That said, I think you will definitely find some players will get their panties in a bunch if you choose your re-rolls after rolling for all your Reserves. I think your best bet would definitely to check with each opponent before the game to see how they play and adjust your strategy to fit.
You can see from this YMTC thread the different ways that players play the issue of rolling for reserves:
dakkadakka.com/Forums/tabid/56/forumid/15/view/topic/postid/104399/Default.aspx