We put our logo & number on the back of everything we can. We manufactured it, and we put our stamp on it, plain and simple. Just like any other manufacturer on the face of planet earth. Always on the back, or out of the way, unless it needs to be visible per town/county permitting requirements.
Also, you would be astonished to see how many people cannot remember where they got their sign made or vehicle wrapped. They get a sign made, a couple of years pass and they just can't conjure the name from memory if someone asks them.
Lots of developments we do work for down here have multiple divisions, and they don't always communicate with each other...so yeah, having a small tag on a sign or banner helps a quick look at any sign will tell them exactly who to call. For wraps we put a decal inside the drivers side door.
I'll take word of mouth advertising all day long, but if it's one thing my advertising and marketing background has taught me it is leave no opportunity to promote your business unused.
It's great that some people can proclaim they don't need to do this because they have amazing large accounts and very important client etc... But why would you want ANY business to slip by you, even the little guys?
It's out of little fish that big fish grow.