Warped rotors will cause a shimmy when braking. If it does it all the time, it is something else. Likely culprits include:
Wheel(s) out of balance.
Wheel(s) bent.
Tire problems, such as uneven wear or separated belts.
Loose or worn wheel bearing.

Raise the front wheels off the ground. Spin them while holding your hand lightly across the tire tread to feel for any irregularities. Rotate the tires front to rear. Examine the fronts closely while they are off. You can usually spot wheel damage or tire problems. If the tires are worn in a feathered pattern, you need an alignment, and may need ball joints or suspension bushings.

The wheel bearing is less likely. To check it, while the tire is off the ground, grab it at top and bottom and try to rock it in and out. If it has any noticeable play, the wheel bearing needs to be adjusted.