I got the same advice to send our application in and wait for the request for the FBI certificate. But here's what we are doing, for a variety of reasons.
I got an FBI certificate, but because of timing with my medicals, it was going to expire before everything was ready to be was sent in, so I re-did the police certificate. But unfortunately that was after the FBI started having issues, so we were waiting. And waiting...and waiting. That's when I got the advice to send without the current certificate.
Then I went to Canada for a visit around Valentine's Day. We were going to send everything in with the old one, and I realized my wife (the sponsor) had gotten something wrong when she did the Option C request; she doesn't get along with automated phone systems and her iPhone. So more waiting. In the meantime, I see that the FBI has charged my card. Whee! Then she gets an Option C, but it's for 2014, which hasn't been filed yet, and of course was blank.
So she talks to a live person and and requests the right Option C, and I'm still waiting on the FBI report. Then she gets the right Option C, Wheee! Again. And I call the FBI and find out it was actually sent in the mail (took about three weeks after the card was charged). So at this point, we're just waiting since we should be able to send everything together and it will all be current. I should have the FBI document tomorrow or Saturday, will express everything to her in Canada, then she can replace the expired one with the new one and she will send everything in. The timing just seems so tight with the three month expiration and I wanted to play it safe...plus we had to wait on the right Option C. At this point, a week's wait seems worth it when we are sure we can have everything be the right documentation upfront.