I am a former machine gunner.
With the FN Minimi (m249/SAW) i didn´t move my weapon sideways during firing. I shot with it like with a rifle, but with bursts. Just took a aimpoint and fired at it. Lenght of burst depended on situation. If i fired at enemy, 6-10rounds. If you didn´t have an exact position of the enemy 8-20round bursts. The recoil will help you spread the bullets. =) Then if you are about to supress the enemy, i used 8-20 round bursts and moved my aimpoint around after every burst.
With the Fn MAG (m240). I shot more suppressing fire aproximatly 8-12 round bursts on an fixed aiming point. Sometimes (becouse you actually can with this wonderful weapon) i moved the firing sideways (mostly) to cover a larger area. I would offcourse use shorter 3-4 and longer 20+round bursts sometimes. This is a really great weapon!
Btw, we only used Bipods to our mg:s.