Dashes beat underscores
My new addition to Mac OS X text replacement:
Replace underscore with em-dash.
This way if I miss type a dash (missing out the option from shift-option-hyphen, which is not unusual) then I will still get a dash.
I use dashes quite a lot, but very rarely use underscores. If I do want to type an underscore… I’ll work that out if it ever occurs.