I recently had a customer comment on the great quality of my grips so I though I would comment a little on the topic. Cork is a beautiful thing in its most natural form and is warm to the touch. Typically I look at them very carefully. I do not use fillers in my grips simply because I do not have to.
If you have ever seen cork grips that look like they should be hanging off of a fishing net in the middle of the Atlantic look a little closer at many of the rods sold at retail today. Many grips with fillers look like the grip has been extruded out of a sausage maker.
The above photo shows what a premium grip looks like after
going through the extra step of locating the lenticels, numbering the
measured rings, and placing them in sequence. Thus avoiding them
during the final turning process.
I start out by using the best cork I can get and then take it one step further. I measure all the clusters of lenticels in the rings to miss the final surface diameters of the intended grip profile. By doing this I avoid the interruption of lenticels on most of the surface area. Filler free is my motto... always.