In our club, we perform a shomen-uchi from the back hand and very very rarely from the front hand. An example would be we start in hidari-hanmi and strike and end up in a migi-hanmi stance (and vice versa).
However I recently attended the BAF Summer School and Sugawara Shigeru Sensei made a comment about shomen-uchi. He was saying (from my understanding) that a shomen strike shouldn't be a strike because when the hand is raised at head height, it leaves an opening for the tori to strike at the face. Instead it should be like a Tsuki to the face, almost like the way Yoshinkan Aikido performs Shomen Uchi Ikkyo Omote.
A example is in the below link, more when Kanetsuka Sensei performs the first ura technique.