I have also seen χτες used, is there a difference? Which is more common/acceptable?
No difference at all. εχθές, εχτές, χθες, χτες are all common and used.
I wrote χθές with accent and DL told me that I had a typo and suggested χθες.
Is it OK without the accent mark?
Monosyllabic words get accent marks only by exception. Χθες should not be accented.
Awesome, thanks! BTW, I didn't know there were good trolls out there. :)
...right, this is going to be one of those fun ones to learn how to pronounce. At least the movement between Χ and Θ feels relatively natural...