I think you have to do more than just keep it open. I think you have to actually use it some (more than the "first purchase") that earns you the miles.

What I've typically done is used new cards for a while for one purpose, say with a new airline card used it for that airline's dining program. Or used it in the case where it gives bonuses (for example, since I already have one card that gives me oodles of miles for supermarkets and drug stores and gas stations, but not home improvement stores, when I got my Delta Amex I used it for only home improvement stores since no other card gave me a bonus there).

If you don't want to "waste" miles, I would think that more smaller purchases would be a better way to show "use" than one bigger purchase, especially if you can spread the smaller purchases over a few statements (so when they review the card they'll see you made payments over several statements).

But, again, it's obviously easier to not get "bladklisted" if you're being "moderate" about how often you apply for cards.

But in regards to keeping it open, yes, I've also heard some cards (but don't remember which offhand) were telling the airline to yank the bonus points if you cancelled the card right after the bonus points posted. I think the general consensus is that it's safer to keep a card at least 6 full billing statements before cancelling it, and I prefer to wait until the annual fee either posts or is about to post, and if there's no annual fee ever, I don't even tend to cancel, I just eventually consolidate if I end up with more than one annual fee card I don't need at a particular CC company. (But again, all this is way easier if spread out over time, in my case years!)