You can turn off iOS automatically converting dashes to em-dashes. It also turns off smart-quotes which when used converts any sms you send from normal GSM-7 (7-bit) encoding to utf-8 which doubles the number of sms messages you're sending in the background (even though they're stitched together to look like a single message)

To turn off Smart Punctuation: Home > Settings > General > Keyboard > Smart Punctuation > Off.