Less CSS Mixin for positioning with options for corresponding top, left, right, bottom values
Is this a useful Mixin? Depends on if you need it a lot.

Or is it a bad practice to create Mixins like this? Nothing is bad practice if it helps you float your boat.

Is it too complicated or too hard to understand? Not really, but seems like you'll often have to lookup the generated CSS to fix positioning problems. F.e. I'm not a big fan of 10px, null, null, 10%, I know it's NESW, but still... Wouldn't it be better to have .position(fixed, top 10px, left 10%) if that is possible?

Can this be optimized or shortened? Probably yes, but why do premature optimization if the use-case is unknown?