Always a good idea to secure loads. I prefer soft cases like these http://froogle.google.com/froogle?q=wrench+roll&hl=en&btnG=Search to store gear/tools in so stuff doesn't rattle around while driving as well as various sized packs.
I have a small bob under my drivers seat where I can reach it at all times, a first aid kit under the passenger seat, then under the back seat is a wrench roll like above with wrenches, another small bag with additional tools, then other tiems like jumper cables, etc. These all secure to the seat belt mounts and seat frame.