Sage advice, as always, from my pal, Terri Vellios.
If you're buying a short sale home, until the sale has been approved by the bank holding the note (and any additional creditors noted on the preliminary title report), you should wait to lock your interest or complete the loan application. Although banks will generally approve the short sale, there are always (it seems) strings attached to any short sale including extra costs, more fees, possible requirements to pay additional money for liens and even, occassionally, a foreclosure from another source that interrupts your purchase. This is why, most of us, advise our clients not to start any activity on the short sale purchase until and unless the short sale has been approved.
As for the credit score issue, I agree that talking with your mortgage broker will be helpful in determine what you can do to increase the score. Keep in mind that, in most cases, credit repairs can take time to affect your FICO score, while, on the other hand, a new credit card or the purchase of a large item could adversely and immediately affect your credit. So work with your broker to ensure that during this "wait period", you keep your score intact and moving upward.