It is very important to include all forms that you receive on your tax return.
The IRS has a matching program, and they automatically review returns to make sure that you report all the income that was reported to them. If their system finds a match to your social security number and the income was not reported on your return, you will receive a notice and a bill for unpaid taxes.
Additionally, since it can take them over a year to complete the process, you can also be assessed penalties and interest for any unpaid taxes, which can add up pretty quickly.