I think funny/personal stickers are fine to add a touch of personalization, but decals of manufacturers are not unless you're actually sponsored by them.
For example, I work in IT and here's mine. Not many people know what it is unless they're nerds lol.
I would really like to find a sticker that implies something along the lines of "my brakes are better than yours" for the people that ride my *** in the slow lanes.