Whenever I've carried posters it's in addition to the personal item and carry-on bag and nobody has ever said anything. A poster tube is pretty small and will usually fit in an overhead that's otherwise full with bags unless they're all the max possible sized roller. If you're on an RJ it's even easier, particularly when one side is extra small.

If you get your poster done early, consider getting it printed on fabric. Print shops are expensive, but Spoonflower.com is an inexpensive custom printed fabric shop that people have figured out can print posters, too. I first found instructions on some astronomer's site, but now they have them on their own site here. Even if you have to pay for overnight shipping, it's less expensive than a fedex store. If your own department has a big printer that someone else pays for, that may be cheaper or comparable.