This problem has been on my mind. Some of my books are in Australian/British English and some, like the Dragon Wine series, are in USA English. This means choosing between ‘arse’ and ‘ass’.
For Dragon Wine I have changed ‘Dragon’s Holy Arse’ to ‘Dragon’s Holy Ass’ but it jars to my ear.
That got me thinking about the music of the two words. Arse has a long drawn out ar sound where ass is shorter and harder.
Stupid arse or stupid ass? Stupid arse sounds posh to me, where as stupid ass just sounds insulting.
This brings to mind people who are not native English speakers swearing. It sounds weird. It would take a lot of practice to get it right. I’ve dated a lot of foreigners and they couldn’t swear (or cuss) to save their lives…in English, that is. It would just make me laugh. But in their native tongue though they could swear but it wouldn’t really register to me. I could acknowledge that they swore but not feel the impact of it.
I recall talking about this while trying to swear in Italian. And it was brought home to me just how insulting an Italian would feel on hearing that particular word. I guess being sworn at in your own language is more impactful.
Anyway that does not entirely solve the argument between ‘arse’ and ‘ass’. Arse is my native swear word but because USA English is very much part of our own popular culture we know and respect ‘ass’. Lol. Hey I respect your ass!
But for me it’s the music of arse that really works for me. ‘Nice arse’ sounds nicer to my ear than ‘nice ass’.
This bit of frivolity was brought to you by burn out. I have not posted in a while as I’ve been teaching. I’ve had a few topics on my mind but this one won the day.
Donna Maree Hanson, Author, Phd in Creative Writing 
The ass/arse dilemma is because the Australian accent is non-rhotic, so our pronunciation of “arse” drops the r-sound to be “a’se”, but when we read “arse” we tend to imagine rhotically, which is why it ends up sounding “posh”.
The best irony is that actual upper-class British accents are also mostly non-rhotic. The common accent we hear that isn’t is American English, so basically when Australians imagine the word “arse” they’re probably associating the pronunciation with Americans attempting to imitate “snobby” British accents!
… yes, this is something I have thought about too much. 😛
Awesome comment. Thank you.