Standard disclaimer: I'm not a lawyer, and I don't even pretend to be one (most of the time). These are just my personal feelings on the matter.

Is it ethical/good business practice to say that I want to own all of the code.

I would tend to look at it sort of like an independent contractor versus employee relationship. The simplest test is who owns the tools. In this case, the main tool is the computer (and also possibly the software). So, are you developing the code on someone else's computer, or on your own? If on yours, I think it wouldn't be too much to ask that you maintain the rights. If on their computer, it would make me slightly more uncomfortable with that.

Of course, no matter how you feel about that matter, you will have to make an agreement with the other party. I heartily agree with Zaxo's advice of a written contract.