I sure wouldn't make a big deal about your toys, but I can't imagine it's a huge issue.
I guess it depends on what you're negotiating and how you're doing it. You certainly won't be able to act like a gushing ******, but that might not be an issue. If you're trying to get them to sell devices to your company at a discount, using a Windows device actually proves the point that they need to do something to get into your office. If you're a component manufacturer, selling them on your widget, then I'd expect them to be swayed by performance, which may require a Windows device to demonstrate. If your company primarily makes Windows software and you're trying to get access to their APIs to right games for the Mac, no big because you're explicitly on Windows moving toward Apple. If you're selling furniture, it's probably a non-issue, too. AFAIK, all the iCloud servers are Microsoft and HP.
On the other hand, if you're there to do a presentation on how well your stuff integrates with Apple products, it might be bad form to use PowerPoint on a Surface. Web design might be a coin flip, depending on how "core" the site would be to Apple business.