From postings over the last few years here on CC, it seems that everyone who is not offered a merit scholarship is asked for senior grades. It is best to send a copy of your grades AS SOON AS they are available because most of the time, the decision on your application will be held until those grades are received.
If the grades "suck or if they are not on par with your other grades", this does not mean you are doomed. If the rest of your application is good, you would probably be OK.
BTW, this is the GENERAL mailing of USC letters:
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End of January through February: Acceptances for those who will receive one of the top merit scholarships- Trustee and Presidential. Letters/invitations for these merit scholarship interviews are sent shortly after those acceptances.
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End of February: Acceptances and letters for other merit scholarships.
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Beginning of March through the beginning of April: Acceptances for all others.
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1st or 2nd week in April: Rejections

(No rejections are sent prior to this time.)