I have no "set" rules.
Each location and shoot is evaluated for the viability and saftey of flight each time.
Some things I consider when evaluating a location.
1. Unsheltered people:
Nobody in the flight path that is not protected by a piece of something more solid than the helicopter. Example Cars, buildings, overhangs, etc...
Exceptions to this would include models that are required for the shot and are COMPLETELY aware and briefed on the dangers and saftey procedures involved. Even then it better be the ONLY way to get the shot and the shot would have to be a must have for the client.
2. Emergency landing zones:
Sufficent to accomidate the helicopter with at least 200% of the normal landing area for over shoot. Normally I say a 10x10' area is a nice sized LZ that affords you some room to move around with the wind and such. So an emergency landing site must be at least 20x20' area. This is usually not a big deal and is certainly adjustable based on the conditions and such.
There must be a clear take off, departure and landing path that does not overfly any unsheltered persons or cause disruption to persons/property not involved with the shoot.
In otherwords no straight up 50' then over the power lines kind of stuff. Too easy to get into trouble especially during landings.
Don't overfly the outdoor cafe and get the customers all nervous.
4. Sun Angle:
Both for the shot and for the pilot. I always look to make sure my flight path is not going to require me to fly into or near the edge of the sun to avoid a loss of orientation issue.
5. Spotter/ground handler:
A must in my opinion on ALL shoots where there are bystanders present or possibly present. We all know the reasons for that.
Those are just a couple of my "biggies".
I look at lot's of other personal preference issues like the terrain, slope, wind directions, swirling winds, surface types, adjacent structures, disturbance issues, etc...
One thing I always throw in the truck is a roll of caution tape. So I can barricade off an area if needed to keep people out.
This can sometimes create those "safe areas" that I consider.
Fly Fast....Live Slow...