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The level to which requirements may be kept up-to-date sometimes varies from company to company based on methodology.  For instance, some agile teams don't believe in keeping user stories once the features are implemented. They say, "the code is the only documentation needed". I disagree with this as I think most analysts and IT professionals do. I believe it's a misunderstanding of the intent of Agile.

First, let's be clear that requirements aren't just text based.  Anything that documents WHAT the system should do is part of your requirements documentation.  Depending on methodology used, this might include "the system shall..." type statements, data models, wireframes, user stories, process flows, or other models.  I prefer to use minimal documentation focusing on what makes sense for a particular situation, but always keep the documentation up-to-date.  

You specifically mentioned wireframes so I'd like to speak to those and other screen mockups and prototypes since I don't believe these should be considered true requirements documentation.  Wireframes and mockups should be used with care as they are an inexact way to communicate requirements. They can be good at communicating visual layouts and concepts. They are helpful for generating stakeholder feedback as part of the requirements elicitation process, but the requirements should be documented in other ways so that the team can be confident that requirements have been documented accurately and unambiguously. Wireframes and mockups can introduce ambiguity as they often unknowingly imply design decisions.  Design decisions are very different from requirements.  They go beyond WHAT they system should do and define HOW it will be done. For instance, if I place a button on a mockup am I telling the development team that a function must be triggered by a button? Is there another way that this function could be triggered? What about the placement of information on a screen? Was it fully thought through? Or, as is often the case, were quick assumptions made during the wireframing process?  What in the mockup is a true requirement versus an unconscious decision or assumption?

Beyond mockups and prototypes, most everything else should be maintained and updated when changes come down the pipeline.  The majority of the work was done when creating it the first time around.  Updating models or test cases associated with user stories shouldn't take much time. And it's this process that will often uncover requirement conflicts before coding of new features or changes begin.