It is best practice not to have an ISO mounted on a VM if it is not necessary. This is especially the case with VDI: if you forget to set the optical drive back to client, each of your desktops will have the ISO mounted as well, which can create a hassle with dependencies on the datastore where the ISO is located.

With PowerCLI, you can quickly resolve the matter with this one-liner:

Get-VM | Get-CDDrive | Where {$_.ISOPath -ne $null} | Set-CDDrive -NoMedia -Confirm:$false

For VDI, note that this will not work for replicas and master images containing snapshots, as the dependency remains intact if a snapshot exists where the ISO was still mounted. In that case, it is better to clone the master image to a new VM, remove the link with the ISO and then recompose the desktop pools.