Usually down the middle for doubling on vocals, but if they aren't blending well (they want to layer two lows for example, and it's just a big war for that frequency range) I'll do some more EQ on it and maybe pan the second layer EVER SO SLIGHTLY to one side so that it sticks out more in the mix, but it's not obvious that it's not right down the middle. Can always do the same on the opposite side if you feel the balance is off. I do the same with lead guitar too sometimes, especially if there's a lot going on down the middle at the time (vocals singing over top of the solo, or maybe a lot of snare activity at the time).