I agree with fel3 and vmirat about keeping all pertinent documents, drawings, calculations, but i disagree with ztengguy about the client needing to look for another engineer if he does not produce the calculations. I would hold on to my calcs until absolutely needed, and that moment would be in court, when asked to compare with the expert witness' calcs they (the plaintiff hired) provide in court. I won't just hand in my calcs, but i'd rather think of them as my ace in the hole, especially if I am confident in my work. But going to court without any paper trail, no drawings, no calculations is like going to a gunfight without even bringing a knife, but hoping to duck for cover.
On a side note, I once had the owner's field rep who was the site safety guy asking me about sheeting design calculations, after i gave in my sheeting design, and I said to him that my design bears a PE seal, and i assume responsibility for it, and he is not a PE, so he is not even qualified to look over my work, not to mention the fact I want to protect my work. I told him if you don't feel it is safe, you can hire your own engineer, and have him do the calcs, because I work for the site contractor and not the GC or the owner.
This brings up a question, should you show calcs to anyone who demands them? I know NYCDEP (department of environmental protection in NYC) wants to see them for drywells etc, from a document review standpoint before approving plans, so i'm ok with that. But should you show your calcs other than in court when you have to?