When I got my mortgage in September I had opened cards like that 100k amex platinum offer in January (was looking for houses all during march and on...). They were only a little concerned with the apps, but I was honest and just said I was doing it for the credit card signups. After saying that did not even need to do a written statement.

I did need to write a written statement though as I made a mistake and let a balance go to the bill stage, therefore it got reported to the credit reports that I had 5k in credit card debt, when in reality I paid it off. I had to write a statement assuring it was paid and to provide said statements.