Should I submit the filing fees when I turn in N-400 and I-912 forms or should I wait until if I'm approved for a fee waiver?

I admit that I have never tried this, but I am pretty sure that if you enclose a check the USCIS will come to the conclusion that you are able to afford the fee, cash the check, and deny your request for a fee waiver or simply ignore that request. Make sure that you adequately document your request for a fee waiver. If the waiver is denied, then you will have to pay the filing and biometrics fees.

I agree. I have not done it for your specific application but have never had a fee waiver denied when I did not submit a check with it.

If you qualify for a fee-waiver, then you do not need to submit a filing fee. See Form I-912 instructions below. If you do not qualify for a fee waiver, your application will most likely be returned and you will have to resubmit everything with the N-400 application fee.