The coolant in the motor expands as it gets hot and gets pushed to the reservoir making the level go up. When it cools down, the coolant contracts and should pull it out of the reservoir. And the level should go down. If you just changed the coolant then the level will go down until the air is out of the motor.
You can fill the reservoir to whatever level you want, but if you over fill the coolant it will just piss out onto the ground through the overflow hose when hot and be wasted. Or cause you to crash if it gets on the rear tire.
If it's already near either of the marks, I'm with dricked and leave it alone.