Pre-boil is fine, post-boil, you have a chance of introducing some bugs.
Do you want something to measure volume in your fermenter? Or in the kettle post-boil?
For the kettle, I just use my white plastic brew spoon...I've cut lines into the handle with a sharp knife at one gallon intervals. Works fine and is sanitized by using it to stir occasionally during the boil (I like to push any hops pellet particles back down into the wort that may get pushed up onto the walls of the kettle).
I don't use it in my fermenter, but I would if I needed to, would just have to be sanitized first. (And of course marked for gallons for the fermenter...different diameter than the kettle.)
Edit: Or you could use some rigid plastic tubing. Check an aquarium shop. It's cheap.