Financial institutions ask me this question about posting for guns almost weekly. My question has been what are you doing now? This issue is fraught with problems for the institution. The biggest is if you have posted no guns allowed now and have no problems fine. If you haven't posted and decide to do so you will receive kick back from account holders. Some insitutions have reported major withdraws from people supporting the second amendment. Others have told me that taking the sign down that stated no guns allowed had the direct opposite effect of account holders favoring gun control closing accounts.
The second problem is what state you are located in. All states have different laws some provide a safe harbor if you allow guns into your institution while others will cause a concealed carry permit holder to lose their license if they enter your bank. This is one question where one size doesn't fit everyone.