Logos are fine as long as you have an agreement in place, but you run afoul of a number of laws by using another companies logo without their approval. Although

Logos are fine as long as you have an agreement in place, but you run afoul of a number of laws by using another companies logo without their approval. Although, it seems this happens all of the time with various companies. As for myself I turn down every request for a logo, I like and use your product but I don't really care if you cut me a small break for using it.