The biggest reason to put a steel guide rod in a Glock is to allow the use of non-stock recoil springs. Since the .40 Glocks use the same recoil springs as the 9mmP Glocks, even though they handle higher levels of recoil, many Glocksters choose to put extra-power Wolff recoil springs in their .40s.
I've done this myself. I think it's helpful, since it seems to soften perceived recoil some, but not essential. Your Glock should shoot just fine with the original spring and guide rod.