I have several different screw extractors. If one doesn't grab it, I use a different kind. When I had issues with the sissy bar screws I realized it pretty quick but still stripped out the hex screw.
What I did though and maybe we need to make a separate post on this for everyone so they don't have the same issues, is I tapped the holes prior to installing new screws. The real issue was that HD didn't have the screws I needed to replace so I ended up ordering them online.
I
found this place where you can get polished stainless screws of any kind/size specifically for motorcycles. They are much harder than the soft chrome screws that you often get.
I also found a place to order the chrome screws, but for the money, the polished stainless were actually about the same and were a better option.