95% (if not higher) of the time I DO NOT edit or modify the stats of a creature from the book.
I like using the creatures right from the book and am depressed when I have to do more work. That means goblin-goblin, etc. Very rarely give extra or different powers for my monsters.
Now, that 5% (or less) times that I do modify it is because I have to - something is too powerful but precisely what I want to use or have set up.
Examples I can think of would be I reduced (ended up dropping it entirely when it turned into a near TPK) the DR of werebears the party was hunting. Or increasing general stats of stock lizardfolk so he could challenge a PC's monk, so that it was an actual threat.
But yeah, bothers me when I have to do more effort or change things - isn't that the designer's job?
I am currently working on my own RPG system and I'm still using PF stats from the book with some quick conversion. Even then, I usually do a straight conversion - no wiggling of the stats except where they are wrong/incompatible with my new system.
EDIT:
I just realized this is possibly a major factor for what bothered me about 4e's "modify to fit what you want". I use the same premise for classes (including restrictions for PrCs for example) and PCs as I do for the monsters. I simply won't modify things, most of the time.
In general, I'd rather not use an option as have to go through the effort of modifying it - even if it is a really simple system to modify. This goes double, maybe even triple for monsters given how many supplements there are out there - don't like a particular creature just flip to another page or monster manual and find something much closer to what you want.