They primarily want forms attached to the return that show income tax withheld. Below is the official instructions. If your 1099's don't show any income tax withheld, you do not need to attach them.
From the form 1040 instructions
Assemble any schedules and forms behind Form 1040 in order of the “Attachment Sequence No.” shown in the upper right corner of the schedule or form. If you have supporting statements, arrange them in the same order as the schedules or forms they support and attach them last. Do not attach correspondence or other items unless required to do so. Attach Forms W-2 and 2439 to the front of Form 1040. If you received a Form W-2c (a corrected Form W-2), attach your original Forms W-2 and any Forms W-2c. Attach Forms W-2G and 1099-R to the front of Form 1040 if tax was withheld.
Hope that helps.
