Credit scores are based on the amount of credit you have vs. how much of it you are using. Do not close all of your accounts! If your credit limit, or "available credit", is zero, that's not exactly a good thing. You want to have a credit limit that is hardly being used (I think less than 7% or less usage is the estimate I was given for "responsible usage).
It also tends to look better if you pay them down steadily, rather than paying off the whole thing at once. The idea is to look "responsible", not desperate. If you spend the max and then lump it all off at once, it doesn't look as responsible as someone who has a long history of paying it down steadily on time. Building a history of paying off on time is good. If there's nothing to pay off, you're not building that history.
I've read my credit report twice over the last two years, and both times I've gotten score penalties for using 50% of my limits, and not having a long enough history (and I've had my accounts open for almost five years!).
Also, if you pay a card off to zero, the lender can close it within a year on you for no usage, which also lowers your "available credit". (It's GOOD to have available credit... It's bad to be using over 50% of it! -- two different ideas).
Good luck buying your house! I hope you find a good deal! I hope to be in your shoes in the next three months, myself. ;)