Personally I think it's evident that the item contains nuts when it says nut in the title. Apparently some people aren't convinced unless they see that sign.
Don't know about that. I found 'may contain' warnings really helpful when I was working in respite care with people with learning difficulties. We had a well-researched list from his mum and plenty of time to go through the biscuit and bread cupboard (it was a great place to work, could have been a rushed nightmare in other facilities), but that warning was really easy to spot and we knew right away it was going into the locked pantry.
We also had to be concerned that none of the other people staying with us that week gave him anything he couldn't have. You'd be surprised how tricky it can be to spot the word 'nuts' in the stylised logos of some product names. Everyone knew to check with us first, but more of them learned how to spot a fine print 'may contain nuts' than anything else.
Legislation requires that the allergen ingredients be included somewhere on the packaging, but the 'may contain' warnings are voluntary. Given my experience in helping keep someone with a nut allergy safe, I think it's worth doing.