The risk in adopting a policy of not checking for multiple signatures is that customers may have rights to reimbursement under the UCC if you pay fraudulent checks that have only one signature (and customers might also sue for breach of contract unless your deposit agreements state that you will not check for multiple signatures). The UCC states that when an organization requires multiple signatures on checks, and a check of the organization is signed by only one signatory, the signature is considered unauthorized. 810 ILCS 5/3-403. Because such a check would be unauthorized, it would not be properly payable. 810 ILCS 5/4-401(a). If a bank makes an improper payment of an unauthorized check, the customer can sue and have its account recredited. Continental Cas. Co., Inc. v. Am. Nat. Bank and Trust Co., 329 Ill.App.3d 686, 698 (1st Dist. 2002), citing In re Ostrom-Martin, Inc. v. First National Bank of Chillicothe, 188 B.R. 245, 252 (1995). Under the common law, Illinois courts see deposit agreements as creating an implied duty of care on the part of banks in handling customers’ accounts and in paying checks. Continental Cas. Co., Inc. v. Am. Nat. Bank and Trust Co., 329 Ill.App.3d 686 at 698. 

With that said, customers are responsible for bringing unauthorized checks to your attention. Under Section 4-406, customers must review account statements and report unauthorized signatures with “reasonable promptness.” (The default rule is that a customer has one year to report unauthorized checks; this can be shortened in the deposit agreement, for example to thirty days. Napleton v. Great Lakes Bank, N.A., 945 N.E.2d 111, 118–119 (1st Dist. 2011).) If customers do not fulfill that responsibility, the bank will not be liable unless it failed to exercise “ordinary care.” 810 ILCS 5/4-406(e). (While the UCC allows banks and customers to alter its provisions by agreement, they cannot sign away the bank’s responsibility to exercise ordinary care. 810 ILCS 5/1-3025/4-103(a).) And, the UCC’s definition “ordinary care” does not require banks to visually examine check signatures: in the context of a bank processing an instrument “for collection or payment by automated means,” ordinary care does “not require the bank to examine the instrument . . . .” 810 ILCS 5/3-103(a)(7).