I am a prewasher.
I like to get as much of the chemicals out of the fabric as I can right away since they affect my eczema, I like to check for colour fastness and get as much bleeding out as I can right away and I like a smoother finish so I get as much of the shrinkage out of the way before I start cutting.
My mother always said that half a cup of vinegar (in a front loader, I can't remember her measurement for a top loader anymore) helped cut out chemicals and to set colours. It also gives a fresher scent to me and helps cut out odours. She always told me to put vinegar in the wash if towels had been allowed to get musty. Also, to wash in water as hot as you can, although not to use the hot water often because that will "help" your colours to bleed. The other thing she told me was that the worst of the shrinkage happens the closer you get to "dry" and as you "over dry" so I set my dryer to "extra dry" and then turn it back on for 15 more minutes. By then as much of the shrinkage as I can manage should be out.
Anyway, that's why I prewash and "how" I do it. LOL!
Tara