It is not mandatory. However, it may be beneficial to you in the long run.
If you report the identity theft, the IRS will assign a 6 digit IP-pin (identity protection pin) that you must use to file your tax return. This is in addition to the 5 digit e-file pin. Your IP pin changes every year and is mailed to you at tax season. You can't e-file without it, and if you paper file and don't write your IP pin on the paper form, the IRS will delay your refund to verify your identity.
If someone files a return in your name and SSN, and then you also file, you can be looking at a 6-9 month delay to get your refund. If you are self-employed, this can screw up your SE tax and your social security records. And if you usually claim EIC and someone files a false claim in your name, you can be barred from getting EIC yourself without filing extra forms and going through additional verification each year.
