In a script, it doesn't matter. Nobody sees the script except for the producers. Now, if it actually shows up on screen - that could potentially be a problem. For instance, if you said "Zippo," that reflects a very specific image. Zippo does not hold the only patent on flip-top lighters, but when you say Zippo, we automatically know what you're talking about. Same with Band-Aids and Kleenex. In the script, if a brand name helps the reader concieve a prop or idea more easily, use it. But, you might have to change it in production.