Within the business analysis community, the debate still reigns about whether how the application will look and how the screens will be laid out is technically part of requirements or design. This debate centers around the wrong question.
The right question is “When is the most effective time to introduce visual prototypes into your requirements process?”. My answer: As soon as it makes sense to do so.
Another good question to consider is “What requirements do a prototype or wireframe represent?” My answer: It depends. It depends on where you are at from a requirements process perspective (eliciting, validating, analyzing, or a bit of each), what types of requirements management practices you have in place, and what level of user interface expertise is available across members of the team.