It is safe to use standard load steel shot cartridges in a conventional 2.5in chambered shotgun, provided that it is of modern construction with steel, not Damascus, barrels and that it is not tightly choked.
Most steel shot is loaded with deep, high-density plastic wads and provided it is not fired through a tight choke, there should be no risk to your barrels.
Your friends are referring to ?high performance? steel cartridges, which develop pressures that exceed the standard proof limits.
Only guns bearing the high performance steel shot fleur de lys proofmark, should be loaded with these cartridges.
Always check on the cartridge box to see that those that you are using are suitable for your gun.
If in doubt consult a gunsmith.