Must admit I'm with Dave and only 3-way plastic trim, use the pre formed corners on metal as they can be really sharp and filing them down looks rough on metal.
I've been doing the 3 way mitre for about 5yrs now, before that I used the preformed corner pieces from genisis. The reason for me changing my method was when I was called back to old customers to do more work in another room say 2-3 yrs later, when I noticed every mitre piece had changed colour to the trim. This was on every old job where they had been used that I went back to. Basically they looked a different colour,mainly white, soft cream and beige all genisis. fortunatly not one of my customers pointed it out, but I was still embarrassed so I was converted to 3 way mitre.
My tip for best longer laster mitres would be allways use trims from same batch, dont save offcuts for months and then use on another job with new trim, colours will differ with time if not initially..