I think it depends on a few things:

1.) How big of a CXB would it be? If your MSR was 3k and you CXB 2.5k, they won't be happy.
2.) Have you spent on the card since the MSR?

I would say still spend on the card until you hit the MSR. Covers your bases in case they try to come back and say you didn't meet the MSR and we want the points back.