I actually think it can be helpful to wear SOME of the brand's clothing but also integrate it into your own wardrobe. It's important to show that you understand their aesthetic more than that you can be a mannequin. Even one well-showcased piece worn with your own clothing shows that you can help a shopper choose an item that she can wear with her own clothing. (And you can even use this as a talking point. Most shoppers need to be realistic about how much they can buy, and it's an asset if you are a salesperson who can integrate something new and "Ann" into their existing closet.)
Also -- totally agree with Susie. Wearing an item with your own clothes shows your personality more, too. That's another important thing.