Extra screws are needed. You can get a feel for how it works using just the strap button, but it needs to be screwed down to stay stable enough to keep things in tune.
Depends on the guitar. I had a Hipshot on my Asat Special and there was no need to use the extra screw. On my Tele however I had to use the extra screw.
I've had 3 Hipshots, and all operated better using 3 screws. I tried using 1 screw but the unit would not lay flat under use. Great bender, actually prefer it to a strap bender.