Toilet paper must be a dilemma for the ultralite backpacking fraternity. Is it worth taking double ply [twice as much weight as single ply] for the extra protection it affords?
As an ultraliter, my answer is no - I don't carry TP at all. Just keep an eye out for good big soft leaves like coltsfoot, butterbur, foxglove, comfrey, horse-chestnut etc. Also I'm happy trekking 30 miles a day on a handful of nuts and jerky plus whatever I forage on the route so I don't carry much food - so there's not much input to create the output to begin with!