Almost always!

I like to re-record real Cellos and Violins (although companies like Spitfire are really pushing the boundaries in that world too). Pianos are almost always samples and they almost always make it into the final mix.

It's also possible to get some incredible unique sounds too (not just perfectly recorded pristine samples). Spitfire Felt Pianos is one example (for £2!). And SoundDust Ships Piano is another cool & wonky piano.