IANAL. Some logo usage probably falls under fair use, e.g., if you're writing an article about the company, there's a reasonable chance you don't need any perm

IANAL. Some logo usage probably falls under fair use, e.g., if you're writing an article about the company, there's a reasonable chance you don't need any permission. If you're using it as part of promotional material, you're essentially saying "this company publicly endorses me", which would not be fair use. It'd also be kind of a jerk move since you're using somebody else's name for monetary/etc. gain. Most companies that use client logos as implied endorsements get client's permission, and it's the only honorable thing to do.