Rather than imposing my old-scholl views on this matter, I went to a more unbiased authority, The New York Public Library Desk Reference which states:

Appropriate closings for a business letter include, "Best wishes'" "Sincerely," "Sincerely yours," or "Yours truly." Informal closings like "Yours," or "Cheers" should not be used.

Closings for personal letters are a matter of choice. "Love" is appropriate for those you do love. "Fondly" or "All my best" or "Affectionately" might be right for friends. As with the rest of the letter, the closing should express your own feelings.

Curmudgeonly yours,

Tom