When it comes to commercial or prewritten adventures, as a rule I do not "rune as written." More accurately, it would be safe to say that the shorter the adventure or module, the less changes I will make to it.
As I mentioned in an earlier post today, I've been changing up Castle of the Mad Archmage to suit my group (and me.) In some cases, that may mean adding a Hill Giant where there was none previously, and in others it may mean turning a room full of rat dung into "two fucking feet deep with rat dung!" It all comes down to knowing your players, the challenges they need and basicaly fucking with them.
I do little if any changing of the adventure (besides system conversion) when running DCC RPG adventures, but even there I tune challenges up or down depending on the party that shows up to play.
The one time I ran a large prewritten adventure "as written" was Dwimmermount, back when it was still being play tested but after the Kickstarter had funded. That session is rather (in)famous. Suffice to say, I am an advocate of making adventures fit the party instead of making the party fit the adventure.
Now, if you are one of those fine folks that have an abundance of time and energy to write all of your own adventures, more power to you. The following question does not apply to you ;)
Do you adjust or rewrite commercial or pre-written adventures to better suit your group or do you prefer to run as written, or perhaps somewhere in between?