Scope/timing of scanning - enterprise level deployments

I'd be interested in hearing from anyone who has enterprise level 
deployments of Nessus, and how you handle a few items, for those that are 
able to share:

1) With regulations such as PCI requiring production network scanning -- 
when do you scan?  Downtimes? Daytime, etc? 

2) What has your experience been with outages, overload, etc based on the 
above?  How have you mitigated the risk of overloading network devices 
with sessions, device failure, etc? 

3) What settings as far as throttling/sessions/# hosts, have you found to 
be most efficient (and over what sort of network, fast ethernet\gb, etc)

4) What settings for safe checks, port range, paranoia, thorough -- have 
been most effective as a balance between accuracy / false positives / 
speed?

5) Have you implemented workstation scanning?  Do you scan all?  A pool? 
Rotate quarterly?

6) For those using a distributed scanner architecture -- what's been 
effective?  what did you "do wrong"?  What do you wish you'd done? 
Tips/thoughts?

7) Thoughts on Security Center as a management tool for distributed 
scanning?

Thanks in advance,
Mike

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