For an internship cover letter, should I write "To Whom It May Concern" as a greeting?

Neither...a cover letter should be addressed to a person...it's easy enough to do the homework (Internet search or phone call) to find out who the hiring person or decision maker would be. Sending it with a generic salutation looks like you aren't serious to have bothered to do the basic homework.

if it's a private company, you could call the main office and ask the receptionst what the person's name is. If it is a large public company, there are books at the library which will list all the personnel. Personally, I'd just call and ask. Always better to address the letter to whom you wish to read it. Also shows that you put some initiative and effort into your pursuit.