In certain cinemas and small venues where you see signs saying that tips are the sole earnings of the usherette then you most definitely do. Other places however, may declare that tipping is prohibited. If there is no information to the contrary, nothing prevents you from thanking the person that showed you to your seat with a coin or two.
Evening dress and dinner jackets are still “de rigueur” in certain places for gala evenings. Usually though, regular smart dress is required for your Parisian soirées at the theatre, opera or for church concerts. However, even in summertime, jeans, shorts, Bermudas and flip-flops are to be avoided.