I don't collect sealed either. When I acquire an older game that still sealed, I tend to keep it that way because if something has survived this long without being opened, I don't really want to be the person to do it. I have done it on occasion when the game in question is pretty much worthless despite being sealed. Otherwise, my plan is to sell it and use the funds to buy an open copy of the game which should yield me some money in my pocket.
As far as current gen games go, I keep it sealed until I have time to play it. Like Wiggy said, if it takes a while to play it and by that time the game has become valuable, you might as well sell it and buy an open copy at a discount.
With that being said, if you get an opportunity to open a really old game that's still sealed, it's a pretty cool experience. I bought a sealed copy of one of the Madden games for the Game Gear for $1 at Goodwill several years ago. I opened it just because. It was really cool seeing a brand new Game Gear game with perfect label and manual even though it wasn't worth jack.