If you are simply renewing a card that is expiring, you really need only submit a clear photocopy of the expiring card.(both sides).
An attorney can certainly make sure everything is in order - this will often avoid additional delays caused by requests for missing documents. Attorneys generally cannot expedite cases (at least, I never claim to be able to). Where there is a good reason to expedite a specific case according to USCIS guidelines for expediting, an attorney may be able to make a better argument under these rules than a non-attorney (and may be more aware of the guidelines).
Are you attempting to electronically file the I-90 or file by mail? Also, are you merely renewing your green card or have you had any of your information change, such as your name? Are you a conditional permanent resident? If so, you may not file this form. Are you needing to replace your green card because it has been lost or stolen? These are different factors to be considered in better answering your question.