[Crazy Jetty]

Well, toys are a terrible financial investment to start with. Very few toys ever go on to be worth anything in the long run. And most of those that do takes several decades to gain any true financial value.
Most of the popular lines for collectors, like MU, DCUC, Star Wars, and Transformers never gain any significant value. Those that do, tend to be rare exceptions (Like the occasional Botcon, Foreign, or SDCC exclusive). Or tend to be significant characters. (Beast Wars Transformers had a very limited cast in the show. Show characters tend to be the more valuable in the line, while all the rest, MiB or not, all tend to be very reasonable second hand market prices.)
Plus, I know a lot of collectors like myself who would prefer to buy an opened, but very well cared for older figure, than one in box. If I'd gotten a G1 TF mint in box... well... I wouldn't have the heart to open it. But my nature is I couldn't let it sit there either. So I have no desire to buy it.
Anything newer would end up getting ripped open.

So I say barring a few exceptions you may want to research first, chances are by opening and displaying your collection, you're not going to devalue it by much, if at all.

For my personal opinion, I couldn't care less about the second hand value of what my toys are worth. That brings me no relevence, comfort, or enjoyment. Looking at display of my own toys that looks just the same as a store display, with them all in their packages only makes me sad.
I say free them from their confines, and enjoy them for what they are ment to be!