I am going to go in person with my resume in hand (cold call) to prospective employers. Do I need a cover letter too or are those for when I mail in an inquiry? Thanks for any help!

A general cover letter is never a wrong thing.

Next are you researching the places, to know who the managers are, who owners are and the such ?

And I guess what level of job, if you are going to grill hamburgers it is different from a engineer job at a aerospace firm

A cover letter tailored to the company in mind is highly recommended

(I am in the recruiting business)

I would say, start a draft of the cover letter before the call. Do some research on the company before the call. Then finalize the cover letter after the call so that you can mention the phone call in the cover letter, which is always a plus.