I have an exercise in my writing course on scientific units. The last question is about units of geological time, and it always starts a debate. I favour ka, Ma, and Ga for all dates and spans of time, but I've never gone unchallenged. People like Ma BP, mya, m.y., myr, and lots of other things, and I've heard all sorts of rules for when to use which, and why. The sort of rules you can't quite remember the crucial details of.
Twitter isn't for everyone, but I think it has some real strengths — it's a great filter, a reliable connection finder, and a brilliant place to ask questions. So I asked Twitter, and compiled the responses in a storyboard:
The story exposed a useful blog post, an attempt to standardize (Aubry et al., 2009, Stratigraphy 6 (2), 100–105], another attempt [Holden et al., 2011, IUPAC–IUGS recommendation], and a firm rebuttal from Nick Christie-Blick. Many thanks to all my Twitter friends — one of whom I've actually met IRL!
Bottom line — there are regional variations and personal preferences. There's no consensus. Make your choice. Write unambiguously.