You can definitely have high and low spots on the fretboard -- rather than "fretbuzz" what you will get is notes that don't "bloom" like they should or slides that don't behave well. One way of testing is to pluck a note hard with a finger on the string close to the nut and then slide up the fretboard while the string is vibrating and it should continue sounding and actually increase a bit in intensity (rather than dying out) as you slide up the string. If you have any low spots you'll hear the tone of the string get dull rather than continuing to bloom as you're sliding.
Re: posts about nut being too low - I set mine up so the strings just meet the fingerboard at the boundary between nut and fingerboard. I don't want any space between the string and fingerboard at that point where the nut and fingerboard meet.
Re: posts about nut being too low - I set mine up so the strings just meet the fingerboard at the boundary between nut and fingerboard. I don't want any space between the string and fingerboard at that point where the nut and fingerboard meet.

