I hadn't noticed but it does looked curved in the photo. It's also a good question Mike.
I have my opinions on why the slot may be curved but I've never heard or read a "real" luthier state for certain.
 
There are two reasons I think it may be curved; one being it was cut by the same saw cutting the fret slots and the other being to clear a certain style truss-rod.
( saw could be machine or handsaw with either having depth adjustment set and easy cutting to follow any radius)
 
 
Since I don't install Fender style truss-rods and hand-cut the fret slots there is no reason to radius the nut slot/nut.
 
When I did the refret on my '57 Custom shop Strat I replaced the nut since I also changed the FB radius and it had a radius nut slot the same as the FB. I didn't want to risk possibly filing a flat slot and getting into the truss-rod so...and it's not much trouble to do the nut but I imagine it's tedious working a radius slot from scratch, freehand. Much better options IMO when one has the choices.
 
 
 

The picture of the nut slot does make it look like the bottom of the slot is radiused like a fingerboard. It must be a trick of the lens and shadows.

Ok, I'll bite....so why would some one prefer a curved nut slot over a flat one?

LOL...I use a nut slot file which is flat but I'll sure find out how flat it is when I install the nut !

This is a very cool thread. You got some mad skills Spacey!

Thanks Bob and glad you're enjoying it.
 
I decided to take a break and grab some photos.
Not much to add as they really tell the story and naturally the
result I'm shooting for is the feel I want.
I use both sides of the file and then it's sand paper.
Guidelines help keep me from where I don't want to go. The rest
is just do it and hope like every other step- ya don't blow it.
Really only one rule that runs through me brain when doing this part...
ya can't put the wood back. :)
 
 
 
 
 
 
Probably the most important line to me- keeping me away from the fretboard. Hit that wrong-game over.

my very first guitar...
 
was a sears and roebuck.
it had 4 pickups, and a tremelo in the style of a bigsby.
 
i mowed lawns for a whole summer to save up enough to buy it, stuffing the money into a wine bottle.
 
i thought it was the coolest thing ever (i think i paid $60)
 
anyway, the NECK............... was laminated pine.
 
 
after i had it for about a month, it developed a sticky spot on the back of the neck, right around the 5th fret.
 
 
it was bleeding Pine Sap, right through the varnish.
 
LOL
 
 
no matter what i did, it would stay sticky.
i got used to hitting a 'grab your hand' spot when i ever i moved up the neck.
might have had something to do with making me bring my hand up in a more 'proper, classical' position!
 
heheh
 

Isn't it amazing how the memories are so vivid?
While I restored my first I was flooded with memories...and
just so very glad that the neck and body had made it through so many years. A '63 Harmony
bought from a local music store where I took lessons. Fortunately it had a Mahogany neck.
 
And the sense of touch...just thinking how we can feel the difference of a
one-thousandth of an inch difference between string sizes so playing it again was
just a trip...thinking of all the hours I spent with it and records I ruined trying to learn
parts...feeling that neck again from so many (50) years ago. 

Excuse me
 
Dribble

Wookiee
Excuse me
 
Dribble

LOL...
 
Well that was fun. Got my first bolt-on neck profiled. Lots of file and lots of sandpaper- down to 320...but I'll go much smaller.
I know some of the pics are blurred but trying to shoot the profile seemed silly but what
is in the shots is shadows. I use them to tell me how it really is. Light and shadows tell me stuff
that feeling just won't find.
Now I can decide on what frets while waiting for the tuners so I can finish sizing the holes.
Hope you enjoy the ride.
 
Oh...I noticed the template and remembered that I had mentioned making some and would post more when and if I got them completed. Well I'm home alone for a few days, have time to do as I want so things did go good so I'll  take some photos and post them here.