I encourage you to SLOP if not prohibited
GPS seems to be more common today, and the accuracy this adds to navigation is incredible (.04nm after a North Atlantic crossing). An eye-opening event seems to occur about once every couple of years for me. Flying on a published airway or NAT track, another flight passes us from below. Both of us are GPS-equippped, and my radio altimeter comes alive (sometimes GPWS speaks up also). This demonstrates the accuracy of GPS, and this accuracy causes me to believe that whenever it is not prohibited, I will always SLOP when my jet has GPS.
With that in mind, I will always SLOP on the North Atlantic and WATRS airspace. I will SLOP in remote areas of South America where SLOP is not prohibited. Likewise for Africa (I have not been there), especially when in areas of IFBP (most of Africa I am told).
I encourage all of you who operate with GPS-equipped jets to adopt SLOP.