EzekielRaiden
Follower of the Way
Okay so...Pretty much any procedure is fair game as long as the realism criteria is met. Imo. I like sharing DCs because I find they are a good way to make sure my description comes across to the players. I.e., I don't make it sound much easier than it is. But I play in games like @Micah Sweet where they are not given. Both are fine and both fit the fixed world sandbox idea.
Likewise, I'll give the GM a lot of leeway in how the decide. Some specific considerations have been mentioned. But it will ultimately depend a lot on style and judgment.
In any case the resulting play style is very different from a narrative one and does what I want.
If anything will do...
Why isn't this decision arbitrary?
Folks have repeatedly insisted that it is not, in any way shape or form, arbitrary. Yet now that we drill down to it, the answer is...a shrug? As I've said many times, with GM effort, nearly anything can be made "realistic", doubly so when that effort happens entirely behind the black box.
There are, in most cases, a superabundance of paths that are either already realistic purely based on what is already known, or which GM effort could make realistic, albeit with varying degrees of effort. (I imagine BMX Bandit managing to propel himself across a 40-foot gap totally unaided might stretch realism/plausibility too much without a lot of prior effort, for example, but most pathways don't require anywhere near that much work in a fantastical or science-fictional setting.) So...if we've already cleared that through-the-floor bar of "the GM put in some effort to make it realistic", anything goes. Which sure as heck looks arbitrary to me!