I build my books on Sigil with a style sheet (posted online at
http://notjohnkdp.blogspot.com -- Dec 23 post, I think it is). I am very pleased with the KDP conversion for Mobi 7 devices, which except for obvious differences like font face follows the KF8 format very closely.
Far be it from me to second-guess Guido, but I would be a bit nervous about overriding most defaults. I do prefer the Mobi 7 paragraph layout (first paragraph indent and no space between paragraphs) to that of the Fire, so I do specify that.