There have been reports of misfiring, ignition failure and crank no start faults following the fitting of iridium spark plugs to the SC58. The reports are vague, they don't mention which iridium spark plugs were fitted, who fitted them or, assuming the faults were rectified, what the root cause of the faults turned out to be.
For my two penneth and having just replaced the spark plugs on my SC58 on a valve clearance & adjustment service I'd stick with the original Honda specification of NGK CR8EH-9 (5666) nickel electrode and ground spark plugs, I did, that remains my recommendation and
I got them here. My reasoning is that NGK give them a service life of 32,000 miles which coincidentally is the mileage I covered on the last set of nickel spark plugs and where I've penciled in the next valve clearance check service and while I'm there I'll be replacing the spark plugs.
By way of comparison the alternative iridium spark plug offered for the SC58 by NGK is their CR8EIX0-9 (4218) which has an iridium electrode, nickel ground and NGK give them a 50,000 mile service life which means that there is not so much difference in the service life as some think.
NB The discussion here is on CR8EH-9 (5666) and CR8EIX0-9 (4218) spark plugs for the SC58, not to be confused with IMR8C-9H (3653) spark plugs for the SC64 which are similar but different, having an iridium electrode, platinum ground and NGK give them a 100,000 mile service.