I'd say between 2-3mm is ok - I aim for closer to 2mm, but then you have to be careful. If you put purfling in, you can break through the binding into the purfling when you shape the neck, especially if the binding is thin.
You don't want to be a hero and cut the tangs so they fit precisely because the fret may expand when you press (or hammer) it in (due to a difference in the radius of the fretboard and fret), so I think you do want to minimize the amount of "tangless" fretwire flopping around on top of the fretboard.
On this guitar, the binding is ~1.75mm and the purfling was around .5mm. After cleaning it up and shaping the neck, I sanded through to the red purfling on the side of the fretboard in one small spot. In the end, I just put a dab of paint there and moved on, but it's something to pay attention to in your design.