The world has moved on. Normal email etiquette from the Usenet days simply looks weird to most people, and cannot be accomplished conveniently with many email clients.
There are some areas where the old rules still apply, notably the mailing lists of many open source projects. But every employer I've encountered recently uses either Microsoft Exchange + Outlook or Gmail as the company standard. Outlook can manage plain text, but has pretty much no support for the rest of old fashioned email etiquette.
I currently send some office emails in plain text, but only because the default fonts on our Korea office PCs mixed badly with the default fonts on our California office Macs, and going to plain text was the easiest way to stop inconveniencing my Korean colleagues. But I reply to incoming messages in the format received; I don't even update subject lines when the thread morphs (Outlook doesn't follow the appropriate email headers, threading solely based on subject line, AFAICT, so changing the thread name creates confusion.) And I stopped pruning the 20 past messages off the bottom of my replies once I realized that some versions of Outlook show you prior messages in the thread by using the attached quotation spew, not by looking at the prior emails.
I suggest you tailor your email style to the recipient, and consider using two separate email clients, and possibly two separate addresses - one for open source lists and your old technical friends from way back when; another for non-technical people and most younger techies.
On the specific question - plain text emails are OK, but 72 character lines with hard breaks won't make you popular. And messing up quoted text is a bad plan.
My personal policy - Outlook-style emails at work; mutt-style emails at home, with my spam filters prejudiced against html emails, particularly those with no alternate plain text version. When I need to communicate with an html-addict from a personal address, I use an alternate email account.