According to lenders that I have worked with....one of the worst things is all the small store cards....like GAP, Banana Republic, Victoria's Secret, Limited, etc.
It's better to have MC, Visa, AmEx or Discover ...even if you have 2 of each "brand" ...than one at every single store.
Even if they aren't used...they will take down the credit scores because they are all over the place and most of them will increase the limit frequently which is the biggest problem with them.
Also, the cards need to be used. Paying them off each time they come in, gives a good payment history, but does not necessarily up the credit score. It's better to keep a very small balance, so the interest doesn't knock your socks off, but timely payments is the best thing for improving credit scores.