I try to keep my thumb in the middle of the back of the neck, but on slimmer necks my thumb starts to wrap around the top a bit. I prefer a bit fatter necks.
I also feel most comfortable on the Gibson scale and like the neck to be a bit rounded, it's just a nicer feel to it than these flat Ibanez necks.
I used to think Extra Jumbo frets were great, but the more I play the more I just want large or medium jumbo frets.
As said, it's a very personal thing.
I played a guitar with a 45mm nut the other day (an LTD Will Adler), now I am really wondering if that is what I should get.