IO related apps ends up by wasteing cpus , mainly in case of Disk operations, as most languages or codes does not has any Async Disk IO utility . Therefore as to maximize optimization of resource utilization we must try and reduce iowait time of machine .
According to my observation issues will come when you have the disk attached outside the server , i.e. NAS , DAS , SAN , Nfs setups , so be sure which kind of setup you have for the disks ,
40% above I/o utilization is problem . Below 40 % it should work normally , but again lower values are more preffered ones of course .