"But I've found on very large ICs..."

On STK type ICs if they screw down I run a bead between the screw holes. If they clamp down I put the bead in the middle across most of it and squeeze the **** out of it until it starts oozing out of the edges. At that point I am careful not to lift it off the heatsink. Sliding it around helps sometimes if it is large and you heatsink compound is thick, but not so far that it can introduce air in there.

And **** is the word, we used to call it bird ****.

It also makes a good insulator for CRT anode caps, and it never dries. You can usually get it wet and it won't arc if you use it. On projo TVs they are usually siliconed to the glass and need to be replaced once you take them off.