Under the dash? I usually do not use anything unless it is in danger of rubbing on something.
Here are my two favorites, though.
This one is the better looking of the two and I am pretty sure is what comes on most aftermarket efi harnesses.
http://www.waytekwire.com/products/1...ng-Braided-PET
This is the one I use most, if I use it at all.
http://www.waytekwire.com/products/1...Flexo-F6-Split
I wrap the ends with tape as needed to keep it from unwinding. Not electrical tape, I hate the gooey mess if you ever need to get back into it. The magical self fusing tape that only sticks to it self.
http://www.waytekwire.com/item/20890...licone-Rubber/