GoogleGuy explains then differences between using a city name or zip code with local search.
[Link to quote
pleeker, about the city name/zip code difference: if you type in just the city name, we'll use the center of the city as the search. The same goes (roughly) for zip codes, but if zip codes and cities have different centers, then you can get slightly different results.
Note that you can type in an exact address and that will give you more precise answers. I just tried that a few minutes ago, and using an exact address changed my results for searches (and the results were a little more precise). So I'd go for an exact address if you want the best results.