The YLoD fix is only a temporary measure.
While it's interesting to break a PS3 down if you're into building PCs, odds are if you do the heat gun fix, you'll be doing it more than once if you insist on keeping your self fixed PS3. It's not a reliable fix.
Pretty sure the only definitive fix for the YLoD is either reballing the GPU/CPU (not a home fix job) or simply replacing the logic board altogether.
The key scenario when you'd want to do the fix as opposed to sending it to SCE would be to get it working long enough to salvage the data on your HDD if you used it for storing media or you have a ridiculous number of hours invested in your game saves. Starting over from scratch other than your trophies (which really don't help) is not very cool.
The only problem with going through SCE for a YLoD is you'll likely end up with a refurb with a formatted HDD.
The only problem with breaking the seal on a PS3 is that SCE service is not obligated to accept it for repairs/replacement although I'm sure there are a ton of exceptions to this, just like a lot of people have received free repair jobs from SCE on an expired warranty. But any warranty remaining is naturally void.