Using Comboboxes To Spellcheck And Protect Bad Data?

I'm trying to make my database easy as possible for everyone to use while protecting against potential typos and the likes. I then had an idea and wanted to make sure if that was in line with accepted practices everywhere else. After all, just because you can do it, doesn't mean you should.

Would it be reasonable to have fields that would likely to have repeating data across various records to be a comboboxes. The fields I'm thinking of are City and ZIP code, which I would start with some more common ones, and allow users to add new ones while having the benefit of auto-expand as spell checker and reduce possiblity for bad data.

I'm not sure what ramifications that may have on forms beyond the fact that more lookup tables=slower database, especially over the network.

So would you want to make city or zip code or other similar fields combobox instead of a bound textbox?