When I launched my app, designing, writing, building, and testing HTML emails was one of the most time-consuming and painful parts of the process. As a result, I started with plain text emails for the first couple of years. Switching to HTML emails turned into a month long project. It was worth it, but it was painful.
At Postmark, we've been trying to make it easier and recently launched a set of free open source templates based on best practices for the most common transactional emails and an open source toolset (MailMason) to make the process suck exponentially less.
You can preview the templates here. By default they're designed to be dropped into Postmark, but it's extremely easy to adapt them to any service. If you're launching a new app, I'd expect these templates could save at least a week's worth of copywriting, design, building and testing. And they have all of the relevant best practices baked right in.