As soon as possible 
.
I apply for any new card for a specific reason (OK my future USAA one likely excepted) and it's worth moving whatever piece of my financial life that is destined for that card over to it immediately.
I honestly don't think lenders care at all; they have already extended credit, they absolutely want you to use it, and likely even before you receive the card where possible if Amex is any indication for the rest of be industry as another poster intimated too.
Starting Score: EQ 561, TU 567, EX 599* (12/30/11, EX lender pull 12/29/11)Current Score: EQ 648, Wally TU 673, EX 687 (06/15/13)
Goal Score: 680 in all three (01/01/14)
Take the FICO Fitness Challenge
