The answer to this as it is with most things is, it depends.
If you believe that the top tier of players in this draft is pretty fluid then trading back to get two players in the top tier instead of one is a good idea. Personally I don't think there is going to end up being a huge difference between Fultz, Jackson, Tatum, Giles, Isaac, Fox, Ball, Bridges and Markkanen.
If the Celtics make a big trade where they send out 3 or 4 guys and only get back 1 then trading down is a good idea, provided you believe that the top tier is fluid.
If the Celtics aren't able to add anyone in free agency and end up just re-signing Olynyk and replacing Amir with a middle tier free agent on a short deal then our team is probably too young to add two firsts plus whoever we pick in the second.
If Zizic and Yabu come over then we probably have too many young guys to add two firsts and multiple seconds.