Who here would claim that they play ANY TTRPG 100% rules as written with no alterations or omissions?
True for a fair number of Indie games like DitV, My Life With Master, Agon, Hillfolk
For more complex systems, I'll be honest -- I probably make more mistakes that alter or omit rules than I do deliberate choices. I just found out yesterday that my belief that cultural maximums in Pendragon limit Prestige Award advancement is not actually correct. I've been running it that way for 3 years now. So it was an error, and now, because I really do prefer it that way, I guess it's a house-rule!
13th Age is the only game I've run for a long time where I've felt no need to make a houserule. Given its complexity, I haven't made many changes to the way I've run D&D4E either -- really just banning a list of things I find makes the game less fun. No houserules for Call of Cthulhu, Fate, or Numenera, even after running multiple yea-long campaigns in each.
The only reason I'll houserule anything is because it makes the game noticeably less fun. I'm not tempted to houserule for realism purposes or to fit some feeling of what I think the genre is meant to be. And it's a burden on people to remember that something has been houseruled, so it really has to be quite annoying to get fixed.
I think The One Ring is the only game I've deliberately houseruled, and that was just to add more options to events in the Journey subsystem to give players a bit more choice in what skills to use. Oh, and I also added a rule that your Lore skill let you know how many languages you are able to work with, because what sort of insanity is it to design an otherwise really nice system for Tolkien that completely ignores the one thing Tolkien cared most about?