This is very similar to another thread I saw on here this morning.
The main issue here seems to be the need to define boundaries. At my last shop, our policy was that if they paid for design work, it was theirs. If they just paid for the
sign (no art fee), then the design was ours. Their logo was always their logo, but accompanying layout work was dependent upon how they were billed. Currently, I'm in the print dept. of an ad agency doing mostly work for businesses owned by our company's owner. So it's a moot point there.
My advice is determine what your boundaries are and clearly communicate that to customers, so everyone's on the same page and there's no confusion. The main questions I'd ask myself in determining a policy are:
1) How do I protect my company's interests?
2) How do I protect my customer's interests?
3) How do I protect the relationship between my company and the customer?
I figure if you can find a policy that you're happy with the answers to these 3 questions, then it's one you can live with.