I open ALL of mine.
I dont buy toys as an "investment" for resale value...I buy them for the nostalgia factor to remind me of the good 'ol days of playing for hours with my favorite toys. I put them on display up on shelves in my man room (okay, boy room
) in battle stances, fight scenes, whatever...and occasionally pull them down, play with them, re-pose them, and put them back up on the shelf.
While I do keep most of the 'hard-side' boxes (as opposed to the deluxes and such that only come on cardstocks), this is just for packing and shipping in the event that I ever move. Yellowing, fading, and the mythical "plastic rot" i here people dreading is fictitious at the least, slightly annoying at most. Plastic sits in landfills for decades without breaking down, and that is being exposed to the elements, heat, rain, etc. Sitting up on a shelf in a nice clean home these things should be virtually non-existance, and I find the people who worry about them a tad over-obsessed.
I do know a couple people who do this as kind of an income booster, but they normally buy them in lots (i know one guy who bought 15 MP Prime's in order to re-sell them at mark-up later), and there is almost no personal nostalgia involved for them. They see it as a business opportunity only...and I don't really see how anyone could have a "half-and-half" compromise. If you buy a toy for nostalgia value, but still worry about what shape it will be in in 15 years so you can maybe make $50 or so when you sell it...well, I don't really see that as real "nostalgia" if you are willing to part with it later.
Besides, most of us have all seen original G1 toys that STILL sell for hundreds of dollars even in relatively bad shape. I saw a G1 Megs at a swap meet a couple years ago, box was all crushed and dilapitated (sp?) and Megs was missing all of his extra parts (was only the figure and the arm-sight-cannon piece), and he had some serious rusty spots. I laughed to myself when I saw the price tag stuck on it...the guy asking $175 for him. To my surprise, going back the next weekend, I saw that he was gone, the guy had found a buyer for him.