That's not true. I've seen many pianos that require extensive amounts of work and tunings to get them back 'in shape' and 'in tune' after hack tuners and players have 'had a go' at tuning them. You can also damage the way the pins are set and can cause them to 'slip'. Not to mention that if you break a string - then you're in for many more problems.
Leave it to the professionals.
You don't just open it up and start hacking at it. You read a book like The Family Piano Doctor and figure out how to do it right.
Keytarist, yes there is a technique for the tuning hammer. Books describe it, but it makes sense to watch someone, too. I don't know how it is in other places, but piano tuners around here seem willing to share their knowledge-- apparently my guy has more work than he can handle and he says there's always room for more tuners.
There's a school in Boston... can't remember the name... that teaches piano technology among other things. My tech has an apprentice who tunes my students' uprights
No doubt, tuning is a serious craft. An art, even. I've tuned some beaters to get them into quick shape, but I don't try to tune my fancy baby grand. It sounds really, really good after my tuner comes.