The two main reasons I know of that make nail trimming necessary are:
1. If the nail gets long enough to curve under the toe. I know that happens with birds, but not sure if it ever happens to iguanas.
2. When the nails are sharp enough to scratch you during handling. Usually the heavier the iguana, the worse the scratches. From what I remember, young igs couldn't scratch me, but you may have thin or sensitive skin.
Iguanas with needle sharp nails can catch their eyelids and maybe even their eyeballs when they scratch their heads. Many times I've seen iguanas rub their heads and necks with their hind feet. They just sort of casually wipe their faces. Normally all iguanas' nails are blunted by running and walking around, climbing, etc, on abrasive surfaces. But captive igs are usually kept on non-abrasive materials. You can make a stack of broken concrete pieces for him to climb on. Put wide ones on the bottom, and smaller ones on top to make it steady. Glue them together with Gorilla Glue, the foaming type(not as obnoxious as solvent-based glues). Slightly dampen whatever stones you use with water to aid curing and adhesion. The stones don't need to fight tightly together for this glue to work.