My preview stays paused until I open a new file or something, which is how I would expect it to work, seeing as new file == new render/new settings.
If you don't want the 3d preview and only the network view on startup, create a batch file that automatically does it for you (assuming windows here)
start tgd.exe -no3dpreview
save that as runTG.bat in the terragen directory (ex. C:\Program Files\Planetside Software\Terragen 2 Deep Edition (Beta)\ )
and you can create a short cut to your desktop for it, and set the icon of the desktop icon to even be the same as the normal tg icon. And when you click it, it should start tg2 up with no 3d preview, only the network view.
I do the same thing on my laptop due to its small screen, only i reverse it, I have -nonetworkview set, so only 3d shows unless i switch over to my node view, so the 3d preview is a decent size, and i can still access nodes when i need to.
Also terragen only rerenders the scene whenever you make modifications to it, if you leave it sitting there it doesn't just keep rendering, it stops once it reaches 100%, and starts a new render only when you change settings/view.
also it probably rerenders after node network because it was no longer displaying the 3d view, and discarded it because if you went to node view you probably made changes... /shrug.
Personally the 3dpreivew never slows me down unless i hit enable preview of a population thats huge, and I like it being there so i can have an idea of the scene/see object placement/etc. On my core2 duo and quad both dont have a problem with slow downs.