I've always gotten a kick out of marketing slogans that read "preground feet", "factory prepped guides", and others used to describe many component manufactorers guides for the rodbuilding community.
It has been my experience, that almost all guides need to be prepped before wrapping. With the exception of Recoil RSPGs (99% of the time), manufactorers just don't have the time to pay the proper amount of attention to this area of the guide. The best you can hope for, in most cases, is a sorry attempt applied to the very tip of the guide.
Notice the high "shoulders" on this guide fresh out of the package. The grinding on the tip isn't ramped down to the base. It extends only half way down through the material comprising the front of the guide. How in the world would you make a smooth transition from the blank and onto the guide foot with thread if you didn't shape this foot? Imagine the tunnels that would be formed on either side of the guide due to the sharp edge of the shoulders (ie..this means stress on the edge and instability of the guide laterally). Also, notice that the guidefoot is not "dome" shaped to mimic the curvature of the blank it is being attached onto.
After only a few minutes of prep, you now have a guide that will sit perfectly flat on your rodblank, a guide that allows the thread to transition from the blank to the guide and back to the blank with minimal gap on the edges, a guide that will be solid and almost one with the rod, a guide that allows the first couple threadwraps to transition smoothly from the blank and onto the guide.
Hope this fills in a few blanks on why/how guide preparation is important.
Thank you for allowing me to contribute,
Defjam