Typography affects writing as much as it affects reading. Yet paradoxically, the fonts that help me to write are not always those that are best for reading. For example, for email, I have recently discovered Avenir which practically forces composing short and clean email messages:
In Ulysses I use another Sans Serif Menlo Regular (their default) with the same purpose: to focus attention on every sentence, word and letter
When compiled, Ulysses texts will be rendered in Serif and produce reading flow rather than writing focus.
I have my favorite fonts for ByWord (Cochin, their default) and for Scrivener (used to be Optima although now trying Avenir). Still, when compiling I am using the five typography essentials which mean Serif (e.g. Baskerville) for main text and Sans Serif (e.g. Helvetica) for headings.
This is the joy of working in plain text – you do not need to worry about right typography when composing. Use the font that helps your writing and think about compiling for readability later.