If your rifle is marked that is chambered for .223 it is not advisable to shoot 5.56 ammo in it. As already stated the 5.56 is loaded to produce higher pressures than .223. The chamber geometry of the .223 is different from that of the 556 chamber. This difference helps the .223 round perform better, but causes the 556 round to reach dangerously high chamber pressure levels. Which could cause a catastrophic failure in a .223 chambered rifle.
As far as steel ammo is concerned,it is a matter of preference. If your rifle will eat it, it should not be a problem. You will hear many people say it's crap and just as many people say it's just fine. It is really up to you to decide.
ETA: In the graphic above the critical difference in chamber geometry is the .0427 taper in the 5.56 chamber vice .045 taper in the .223 chamber.That angular difference results in 25 thousandths difference in freebore (less in the .223). The steeper taper in the .223 chamber prevents the bullet from setting as deeply in the chamber when the primer ignites and before the powder ignites. This reduces the volume of space behind the bullet and causes the pressures to rise to unsafe levels.
Read this for a more comprehensive explanation