If pipes are frozen but not burst, is it better to leave main water valve on or off?

I don't think it matters if the main valve is on or off, but I would open the faucets a little. This as the water is heated, it will expand a little, and you want it to have someplace to go. if the freezing didn't burst the pipes, the pressure from heating it probably wont either, but just to be safe. If you leave the main valve on ant the faucets open, then when the ice melts a little the flow will push the chunks out. they may get stuck further up the line, but that will likely be in the warmer part of the house and they will soon melt. when the water just starts flowing a little slowly, let it run for a while and the flow will help melt and flush out the rest of the ice.