Swarmkeeper
Hero
Count me in the camp that uses "progress combined with a setback" but not the "success at a cost" rules.
While only calling for ability checks when there is a chance of success or failure AND a meaningful consequence to failure, sometimes the situation calls for allowing the PCs to make progress but with a setback. Success at a cost is too fiddly - I like the simplicity of pass/fail and I can assign the appropriate level of consequence on the back end (which I will typically tell the player before the roll anyway).1) Why do you use it if you do or why do you not use it if you do not?
Yes. I played 1e then took a couple decade hiatus from RPGs.2) Is this the first game you've used this GMing technique or did you use it in the past in other games (and when did you first use it)?
Certain instances.3) If you use SWC or FF, do you use it on every instance of action resolution or only certain instances of action resolution?
The bottleneck example provided by @overgeeked is a good one. Even on a failure, let the players proceed through the check to advance the story but with a setback of some sort that challenges them.4) If you only use SWC or FF on certain instances of action resolution, what principles/reasoning underwrite your decision to use it here, but not use it there.