If you think it's confusing, as you've said, then don't do it. If other members of your team that are likely going to need to work with this code don't have the difference between references and objects sufficiently ingrained in their brainsingrained in their brains then don't do it to avoid confusing them. If you all understand the concepts so effectively that this isn't confusing at all, and you think it might benefit you to convey that the reference to the list isn't changed, then mark it as read only.
Keep in mind that your team doesn't always need to do the same thing as everyone else on the planet. Do what works best for the people who will actually need to touch this code (with some basic considerations for people who will need to touch the code that aren't yet on the team.)
