I've read up on this, and myself have cedar soaking right now.
I read that it's totally fine as long as you know it's been well soaked/aged, and there's no resin leaching out anymore. Many people used cedar, without problems. Though it isn't a hardwood, it's quite dense and holds well long-term in water (many cedars are bog/wetland trees). Not sure about pine, though... I imagine that because it's a softwood it'll break down more quickly, but that doesn't mean it's not fine for the shorter term.
Going to test mine in a 20 gallon tank with a few small fish hopefully...