That's a really good question that I don't have a definitive answer for, yet.
Perhaps it's my own capabilities, but I've found the D800e to be very sensitive to any movement. The 1/focal length rule seems to fall apart rather quickly. For instance, I have an 18mm f/2.8 lens and shooting at at 1/18 sec doesn't result in a good image. Even a 50mm a 1/50 doesn't work for me.
Most of the shots I took on this trip were in broad daylight, so keeping the shutter speed up was no problem. Even so, I ended up with more blurry images from camera movement than my normal. Unless it is pretty bright, ISO 100 seems to be strictly for on-tripod use.
Yesterday, during Puget Sound Chapter outing, I decided to experiment by using Auto ISO and a minimum shutter speed of 1/160, max ISO 6400. Just about all of the shots were keepers from the point of view of sharpness and lack of motion induced blur. Here's one of the shots:
I think I will keep experimenting with this too see how low I can go on shutter speed, and still get reliable hand held results. Also, ISO 6400 on the D800e is pretty noisy. ISO 3200 seems to be a better upper limit.
VR helps. I had a few shots with the 24-120 f/4 VR at 45-55mm that maxed out at ISO 6400 and the shutter speed dropped to 1/40. If you discount the noise, they are sharp.
I need to look a little closer at the images I have taken and get a better feel for the correlation between my ability to shoot a a given shutter speed and getting sharp images. For now, I'll be conservative a keep the ISO up to maintain a faster speed.
On the other hand, I am much more willing to shoot with a slow shutter with my D3s.
I don't think my technique is all that great. I think it is just a question of keeping the shutter speed up and bumping the ISO as required to do so. Auto-ISO certainly helps. The camera is pretty good with noise up to ISO 3200.
So far, my impression is that anything less than 1/125 sec is too slow with a non-VR lens, regardless of the lens focal length.(At least for me.)
Mick have you tried burst shooting? With my D700 with very low shutter speeds hand held, I find that can solve the problem as you will get at least one image that has no motion. You are making me glad I did not upgrade from my D700 as I am shooting very often at 6400 and slow shutter speeds. That said I really like your images as they are wonderfully graphic. You have opened my eyes to possibilities in Portland which I visit occasionally.
Portland is a target right environment for a photographer. Compact, so walking is no hinderance, and the transit system will get you most other places.
I haven't tried burst shooting, at least on purpose. It is probably worth a try. Using a tripod with the D800e at low shutter-speed is probably more intelligent, especially given the file size.
Trouble is, I tend to be a "walk and shoot" kind of person. I'll probably us the D7000 or even D3s more for this. I really bought the D800e for when I want great detail, and have the time and space to work on a tripod.
Watching the "Portlandia" TV show can give you a quick overview of what Portland offers. I have a nephew living there, a transplant from the East coast. He likes it but all the rain in winter tends to depress him. I go visit him now and then but have mostly focused on the nature in the area, so have not done that much walking around. At least your D800 is a bit lighter than your D3s. I rarely use my tripod unless I am driving, as I hate to haul it around even though it is really light. I can hand hold my D700 with intense concentration down to 1/8 sec without VR as I only own one VR lens. But I tend to do that with my wide angle and short telephoto lens not my 70-200 or 300 mm lenses. for those I keep the shutter speed up. I mostly keep my camera set to Auto ISO and have set three basic shooting modes, portrait, Landscape and action. I vary the max ISO for each and set different lowest shutter speeds I will tolerate for each. for example I keep my min shutter speed much higher for action and let ISO float to 6400.