The Ashigaru were the lowest of the low(many of them were commoners) and usually not considered to actually be Buke, thus it was not expected of them to adhere to any code of Bushido.
The Hagakure is a collection of short notes on everything from the correct way to perform Kaishaku to stories about adultery and make-up advice. Some of these were treated as guidelines and/or advice by the Samurai of the Hizen province. With its fanatic "dont think, act!" apoproach it contradicts earlier works like the writings of Yagama Soka, who promotes wisdom and ethics.
edit: Wiggin, ye darn nitpicker. of course I was talking about "lowest iof the low" within the warrior class :ph34r:
interesting note: The Hagakure actually dismisses the actions of the famous "47 Ronin"; an event who is usually heralded as the epitome of Bushido display. Yamamoto Tsunetomo argues that they should simply have rushed headlong to attack their enemies fortress and been cut down, instead of waiting two years to plan it so that the operation actually had any chance of succeeding..