There are a few things to consider when answering this question. First, it depends on the type of leather furniture you have. If you have a softer, more delicate leather, then your dog's claws could definitely ruin it. However, if you have a tougher, more durable leather, then your dog's claws probably won't do much damage. It also depends on how often your dog uses his or her claws on the furniture. If they're constantly scratching at it, then it will definitely start to show wear and tear over time. However, if they only use their claws occasionally, then the damage will likely be minimal.
In general, though, it's best to err on the side of caution and not let your dog use his or her claws on your leather furniture. There are plenty of other places for them to scratch (like a scratching post or toy), so there's no need to put your expensive furniture at risk. Plus, even if the damage is minimal, it can still be unsightly - so it's just not worth it!