The myth that letterheads are only for large companies, accounts or lawyers who can afford them is today forgotten given the advancements in technology and reduction in print costs letterheads are now affordable to all in business.
Letterheads are an effective branding tool and should be used on all forms of your companies documented communications like:-
This assists with brand recognition and you'll often find accounts people identify easier with prettier looking invoices (which may actually help with getting paid ontime!!) and in general when customers identify with your brand it's easier to draw them in for a sale.