There is only one way to safely flush a slide stop. You need to be very aware of the way the pistol functions and how loads are transferred to the frame.
I side with Bob on this one. When it is done it needs to be done correctly. I would not care to imagine the repercussions if done poorly on a 10mm.
That being said when I do it it is done with a carbide 3/8" ball end mill. I only go .060 deep into the frame.
That way you have:
1-An evenly radiused depression.
2-Controlled removal of frame material.
3-Minimal impact on frame strength.