While SLES for SAP is not required it is highly recommended for the following reasons:
Several kernel parameters including memory optimizations differentiate SLES for SAP
The SAP applications are fully tested and benchmarked on SLES for SAP
Troubleshooting and performance issues can benefit from better and timely technical support
If you are a certified SAP installation engineer or a certified SAP architect you can probably follow the SAP best practices and perform modifications on a standard SLES 11 implementation.