When typing a letter, is the "Sincerely" part of it indented, or left justified?

The answer depends on your overall letter style.

In the block style, everything is left justified. So, your letter might be:

February 3, 2009

Dear Sue,

Hi, how are you?

Sincerely,

Lizzie

Many prefer to write their letters with some indentation. Basically, if you indent anything you should indent the Sincerely and the signature.

So, you might indent just the date and the Sincerely, etc.

February 3, 2009

Dear Sue,

Hi, how are you?

Sincerely,

Lizzie

Another style is to indent the date, the beginning of each paragraph and the Sincerely, etc.

February 3, 2009

Dear Sue,

---Hi, how are you?

Sincerely,

Lizzie

[I can't format correctly here. Imagine that the dashes before Hi are spaces. The point is that the paragraph indent would be less than the date and closing indent.]