The processing afterwards may change the way you wanted to fade... If you use mix compression and use a fade that is slower than the release, the compressor will even out your fades. BTW, this is just one of the things, mastering engineers were considered to be doing originally.
There's an awfully lot of very well respected engineers who disgree with Bob Brockman about the need, both technically and musically, to record super hot at 24 bits.
I guess we'll have to agree to disagree on that one.
In general it is better to let the mastering engineer provide fades so that we have room to work. You can't add back something that has been removed if you decide later that a fade was too short. Sometimes a reverb can be added in this situation, but it isn't as natural. It also allows you to work with crossfades better if you want to fade one song into the next.
After compression/limiting the noise level is raised in the ins and outs. On intros I don't like to have things come in too abruptly and try to keep a little "air" before the first note/drum hit. In order to accomplish this I like to fade in slightly before the the music comes in while removing any background or other types of noise. If someone just chopped this off it wouldn't be possible.