Yes! Be 100% forthright and honest with your attorney. First, make sure you vet your attorney before hiring them. But once you are working with them, it is their job to help assess everything about your case and do their best job to represent you. If there is information that comes out during a case (or trial) that they could have known before, it could undermine your entire case.
You are better off telling your attorney something you think may hurt your case so they can prepare for it, instead of letting them be caught by surprise later.