How should we sling a M4/M16?

This depends so heavily on the situation you're in.  For example, there were times on patrol that I slung it across my chest, over the top of the rest of my kit.  There were other times where I used a dragon sling and had the weapon hanging off my firing side.  People on here probably have countless other examples of how they approached it.  My unit didn't really regulate how you did it.

Low ready at all times on deployment.

In Garrison? Whatever the heck 1SG/SGM wants. It's easier that way....

I think you need to clarify the setting. Is it a foot march? A tactical patrol? Walking around a FOB? All would have a different method in my opinion.

In Parades, the way they told me, in the field/deployments- any blasted way I wanted to t, in order to be able to react quick as possible.

This should be variable- so that the individual warfighter can safely and rapidly employ their weapon while maintaining positive control of it. Not every shooter is comfortable with the same techniques and there are various roles on the battlefield..individual shooters functionality has to be taken into consideration. If they are comfortable (within reason) they can fight better and limit distraction.

During both my deployments, I used a "Mamba" 3-point sling for my M-16 or M-4. When not wearing body armor, the sling kept my weapon at about the height the low-ready would be executed. When wearing body armor, I would use a carabiner attached to the right shoulder of my IOTV to clip the butt of my rifle to my body armor. That made the rifle hang at the height where the high-ready would be executed. Both were comfortable for me and kept my hands free to do other things, but always had my weapon one motion away from a ready position.

Granted for ceremonial purposes it should all be uniform. However, outside of that, I believe shooter preference should be the standard. LTC (Join to see) If you dig into FM 3-21.5 and look at the Appendixes, it gives photo examples of the weapons at various positions, including sling arms.

Come on soldiers - weapons safety? Always piont your weapon into the ground in case of neglegent discharge...

It kinda depends on where you are and what you are doing, wouldn't ya say? I think absent an SOP, what ever is safe and effective.

In basic training, we were taught to always wear the rifle on our fronts pointing down. Typically when carrying a weapon at port arms, it will be pointing up; sling side arms is muzzle up, unless it is raining. You will only ever wear your weapon on your back if you are part of a working party or are otherwise occupied and cannot have the weapon on your front. In this case, it will be muzzle up, unless it is inclement weather (such as rain), in which case it will be muzzle down. Other than port arms, when your weapon is worn with a one, two or three point sling on your front, it should be pointing muzzle down with at least one hand on the weapon at all times (positive control of your weapon).