The answer to that question lies more in the nature of the business or job and how they do the hiring process. The answer also depends on what the company prefers. You need to research each company or business to find out.
Submitting resumes by mail along with a cover letter is quite professional and often is preferred. Please recall that these people are busy and may not appreciate someone barging in with a resume unannounced. Doing so would be very unprofessional. Also, if a company doesn't indicate that they are hiring, submitting a resume likely is not a good idea. In such instances, one should send a letter of inquiry if one is interested in working for that company.
There are also many companies that prefer that that folks submit submit cover letters and resumes online. On the other hand, there are sometimes hiring fairs, or a company is expanding or just opening. In situations such as these, arriving in person with cover letter and resume during the posted times would be quite appropriate.
I have always tailored my resumes and cover letter to the specific job I am applying for.