I recently went back to playing my Boomkin as an alt, and I assumed everything was the same as in 5.1 and 5.2 (ie start at 75 lunar). I read up the 5.3 guide here and it said that. However, I spoke to our raid's Boomkin and he said that sometimes he starts at Solar, specially in multi-target fights. Normally I'd laugh, but he pulls really good numbers, so I dunno.