I'm gonna be honest here - it's your money, do what you want with it. Want to open it? Do it. I personally wouldn't, as there are plenty of loose figures in circulation, that can be had for cheaper than the sealed for sure, but if that's what you want to do for your collection, have at it.
Also...I have no problem with figures that are on absolutely destroyed cards being open too. I saw a "carded" B-Wing pilot that half the card was torn and tattered and water damaged. It was selling for $5 more than the loose one...at that point, just free the damned thing.
I have one bagged figure and I am leaving it sealed. C-3PO with net and removable limbs. Keep it sealed. I don't think it is as bad as opening up a carded fig, depending on condition of the card (to answer your question).