When I prewash, I make sure to wash everything - when I don't I don't prewash anything. A lot depends on color intensity and end use. Lately, I have done more and more prewashing because I got involved with swaps where prewash is a must.
I don't use soap unless it is a full load. (Most of my pieces are run through the sink). The reason for NOT using softener is that some applique iron-ons react badly to softeners.