Yeah a general rule of thumb, preview window will not give exact results in anything but a pose. Especially Poser's preview window. As long as it looks fine in render, then it's okay. If seams still show up in render, then yes, that's a problem.

 From the smith micro website:

OpenGL Preview Improvements 
Real-time preview engine improvements in Poser 8 now display lights and their accumulated values more realistically, sorted by intensity. The light properties control allows the user to select each light they wish to illuminate the scene preview. Mip Map support has been enabled for enhanced performance when previewing large textures. Together, these real-time preview engine enhancements let the user view larger textures and offer more complete scene previewing when setting up lights.

 If you're using Poser 8 the seams are because Poser 8 uses mipmaping in preview mode to save memory. The further zoomed out you are, the bigger the seams appear because of the mipmaping (reducing the size of the textures in preview mode by half concurrently the further zoomed out you are.) 

If a 4000 x 4000 texture that is 20 megs in size is loaded, when zoomed out to X distance it will be reduced by half proportionate to higher X values. So if X = X distance, then the textures will appear at half size (2000 x 2000) and only use 10 megs of RAM to be kept loaded instead of 20. If x = x * 2, then it will be quarter size (1,000 x 1,000) and only use 5 megs of RAM to be kept loaded.

Here's VSS in action. I used hborre's above data to render this in Poser 8. All materials but her skin were set to 0.8 diffuse for conservation of energy. Lighting is one spotlight with inverse square fall off and a radius of 140 degrees at 65% intensity, and a very low ibl to help with IDL (because the 'room' is actually just a corner with 2 walls and a floor, no ceiling or other walls) at 35% intensity.

Render time was 5 minutes and 16 seconds.

 Oh my gosh bagginsbill said my name and said I was correct. Excuse me while I go burn that into my memory forever.

pause

Okay, back.

Was I right to borrow the skin shader for the tongue, or do you recommend a different shader?

 I did not. I just copied and replaced for simplicity testing. The reason I used the shader on it is because it gamma corrects the tongue as well. Before, the tongue looked purple, but now it's bright pink proper.

 Also try putting the same skin shader on v4's tongue and see what happens. I copied the lip material onto the tongue and replaced the texture maps and decreased the specularity a little.

It really adds to the realism of her tongue. It now looks wet, bumpy and much better than before.

Have you set your key light (the point light in the upper right corner) to inverse square falloff? I have, so when I scale back the IBL  intensity I do see a major difference (since the point light has a smaller range of luminance and is not constant).

 No, that is actually incorrect. While yes, using IDL increases render time, it produces a far more realistic result than ambient occlusion. I am a realist type artist, so I aim for realism in my images.

IDL is worth that extra calculation for the results it produces. I always render with it now. Ambient Occlusion is collecting dust.

I'm also experimenting with tone mapping. I've seen from bagginsbill's previous posts that he has used tone mapping in a daylight exterior scene, so I tried that as well, and got some interesting results. A bit too bright, so more experimenting required before I show images, though.

 Another test. All elements gamma corrected via the material room, no post processes. Same lighting setup. 6 minute render time, Poser 8.

 It's high because the ibl image I'm using is a dark grey top facing hemisphere, so I had to increase the intensity for compensation. Similar how in a studio the photographer will concentrate the most light source using the Key light (which in this case is the upper right point light). IBl is fill lighting to remove most of the shadow from key light.

Edit: It's VSS with bagginbill's skin shader handling the gamma correction. Set to 2.2

 That's what is done. I've got a light grey image plugged into the color section of my IBL's material, and I've tinted the Diffuse to slightly, just slightly, yellow. Intensity is 100%. Contrast is 1.

 No, not useless. As bagginsbill has said, if you do not have an environment, using an IBL will create one for you, surrounding your scene with the luminance values of the color map used in the IBL's material. IDL takes that into account, so it was needed here.

Is it possible to use VSS to apply to everything but something? Like everything (props included) but Victoria4?  ( I have a saved character of V4 with your VSS skin shader already applied, and I'd like to use VSS to apply GC to all other materials but her since she is already gamma corrected.)

That should help a bunch. Hopefully. If not, I'll repost here. Thanks, folks!