Actually, NOT putting on grease (or at least some sort of thermal conductive agent) is a good way to help along the early demise of your CPU.
The standard grease which most people seem to use silicone based and is white. The electrically conductive silver stuff is darkish silver and the electrically non-conductive silver stuff is a much lighter silver colour (oooo.... shiny paste....).
The silver (and copper ones) are much better than silicone, but the silicone ones are good enough. Stick with the electrically non-conductive ones though, like the silicone one or Arctic Silver (but NOT the Circuit Works silver stuff).
If I recall correctly, the Radio Shack stuff comes in very large amounts and is cheap and electrically non-conductive. However, it isn't that good. Given that the silicone stuff only costs about 2 deutsche marks (enough for several CPUs) you may as well get that stuff, unless you're willing to spend 30 to get the awesome Arctic Silver (if you can even get it in Germany).
P.S. It may be quite hard to test without using grease. Unless you have a very good chip, my guess is that it will very difficult to have it survive extended stability testing without thermal grease. However, the same chip may be just fine with thermal grease.
This is pure conjecture, but I'm guessing if it boots and loads Windows fine and runs for a least a little while at 1.65 V without thermal grease, it may be fine with thermal grease at the same voltage or a little higher. If it makes it that far, then feel free to put the stuff on to test more. (Personally, the idea of not using thermal grease even for testing makes me uncomfortable however.) Also, you can clean the stuff off with acetone and EtOH, but I can't remember if the any noticeable residue remains.
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