these days, mine seem to usually snap at a fret... my frets are a little rough. These days, I don't finish mine up to a smooth shine. I don't know why but I kinda like the texture... but it wreaks havoc on the strings... after a while there are visible notches on my strings from where they've rubbed on the frets, especially when bending strings... then soon after that... SNAP! time for a string change.
but, yeah, most of my nuts are pretty sharp too... I can see that eventually causing a string to break... but it should take a while... my guess is that unless the nut groove is overly tight and/or there is sharpness protruding into the string, the string should have decent longevity... from my experience, strings are pretty tough. I don't know if I'd recommend slightly rounding the nut there just for string longevity... I have no idea how that might affect the sound/intonation... although, "rounding the nut" does sound enjoyable.