There are times when it matters, and times when it doesn't.
There are also times when it perhaps shouldn't matter but the person reading it thinks it does.
So, for things like covering letters for employment applications, where there is more at stake than getting your point across so that it is understood, it's as well to be able to get it right.
Yesterday I sent my CV and a covering email to an employment agency. I said that I would very much appreciate it if they could confirm receipt.
The reply came (from a Director) with a different font in the text to that used in his footer. I reproduce the reply verbatim:
"Received and forwarded to one of our consultants, who will contact you if they have a position to discuss with you."
No Dear Phoenix, no thank you for submitting your CV, no nuffink!