In my experience it can be quite confusing for the customer if a presentation uses the design company's own branding. There might be a clash of styles and it can seem little smug and out of place to show of your own branding at a presentation.
On the other hand, using the proposed branding for the presentation itself can also get messy. Especially at the early stages where you sometimes need to show alternative solutions. During revisions you'll also have to spend time updating the presentation style which can get really "recursive" and end up being a big waste of time.
For me the best solution has been to create a very minimal style for presentations. Lots of white space, no company logo, lines or other graphic elements (or very subtle). Only mono spaced font or a very simple sans serif in a small fixed font size. It can be black, but sometimes I use cyan because it clearly indicates that this isn't part of the proposed design and it gives the presentation a "technical" look making the proposed design itself "pop".
I try to let the design speak for itself and only add small labels here and there to guide the customer through the presentation. My goal is for the client never to mention the presentation but to go straight into revising the proposed design.