What I was told was that "Dear Sir" should be used when writing to a specific person but they are unknown to you, whereas "To Whom it May Concern" is more appro

What I was told was that "Dear Sir" should be used when writing to a specific person but they are unknown to you, whereas "To Whom it May Concern" is more appropriate if the letter could be dealt with multiple people (e.g. a department) or you expect it to be passed on to someone before being dealt with. I was also told that it's far from a hard and fast rule, and that it doesn't matter all that much.