could just as well

A phrase used to discuss a hypothetical alternative that would have had results just as negative or unpleasant as what actually happened. My book report got such a bad grade that I could just as well have handed nothing in. With the way my parents yell at me when I get home just five minutes after curfew, I could just as well have stayed out all night. I could just as well have not gone to that meeting because the board was not interested in any of my suggestions!

might just as well

A phrase used to discuss a hypothetical alternative that would have had results just as negative or unpleasant as what actually happened. My book report got such a bad grade that I might just as well have handed nothing in.

could/might just as well...

used to say that you/somebody would have been in the same position if you had done something else, because you got little benefit or enjoyment from what you did do: The weather was so bad we might just as well have stayed at home.