Virus scanning is a problem for basically all Windows virtualized environments. Whatever you decide to do on the storage system probably isn't really going to fix the problem. It is just going to postpone the problem to some point in the future. Virus scanning vendors do offer central management of schedules, so you normally do have a chance to group and randomize scanning as a bit longer term solution. VMWare also is working in integrated AV solutions, but I couldn't tell you what works best there.
That being said, setting 'no_atime_update' is technically unproblematic on the filer side. You just need to know if you have systems that rely on access time being updated. You probably should consult the best practices for VMWare, but we use it on NFS-based datastores for VMWare with no apparent side effects. I couldn't tell you off-hand how much of a load reduction you should expect, but I don't expect it to be that significant.
CIFS access probably isn't a terribly large part of your load, but you basically need to know if the file systems need to update access time for some reason before changing the settings.
Hope this helps.
S.
