I try to use the rules and not make it situational, because without that, players would have less of a reason to invest in skills that allow them to be less surprised, or invest in skills that allow them to be sneaky.
One of the long-standing problems that my group would have is "they can't be sneaky". So no-one ever tried. No covert ops roleplaying, no clandestine activites, just a bunch of powerful guys knocking each door over and trumping loudly along on the way to the target.
So, I came up with rules to give my players a way to measure their sneakyness so they can decide when to try to be sneaky and when not to.
What I do is simply require that everyone involved in general sneakyness be taking 10 all the time. I didn't want people who were sneaking up to the fortress to "roll a one" and suddenly stand out like a sore thumb. I also didn't want relaxed guards, just on watch to suddenly roll a 20 and spot even the best hider.
So I created a spreadsheet, set it up with areas for the best spot, best listen, worst hide and worst move silenty for each group, bad guys and good guys. Then, it has options for whether each side is 1/2 moving, regular moving, or in "ambush" mode.
I've used this to help determine encounter distance. (using 10 feet for every 1 difference between the spotter/listener and the hider/move slientlyer) For example, the party was moving through a dangerous badlands, the Blood Steppes in the Scarred Lands, with a great desire not to fight things in the monster-infested area, but they didn't really have a time crunch, so I set them up to be moving carefully (1/2 movement) and trying to avoid encounters (taking 10 on move silently and hide checks). In addition, they using good scouting tactics, and had the best overall hider, move silenter, and spotter be ahead of the group. With all this, and assuming everyone is taking 10, I could use the spreadsheet to come up with encounter distance. On several occasions, the party spotted or hear the bad guy before the bad guy saw them. The party then had the option of stopping and hiding more, and they felt that their sneakyness was paying off.
If they wanted to judge their sneakyness before attempting it, I could outline any numbers on their side, and do what-if's: "If the bad guy has spot/listen equivalent to Bob's you'll likely be able to get to within 50 feet before they spot or hear you. If they are like Tank, you'll probably be able to walk past him in the shadows"
Other times, when the party wanted to sneak up on a group of goblins, and they understood how to maximize their hider's and move slienter's tactics to get there, They basically established a surprise round for themselves, and I was able to set the encounter distance quickly.
Now, all of this involved some time in fully understanding spot, listen, hide, and move silently. And Hide is the worst of the bunch. You need to make sure everyone understands that it is useless without concealment or cover, and by extrapolation, you can assume that without cover or concealment of some kind, spotting someone 200 feet away is NOT DC0 +1 per 10 feet = DC20. If the guys being snuck up on have made sure to have a clear view out to 60 feet, you're not getting any closer than that. If they are a bunch of dumb goblins keeping watch in rocky terrian, you may be able to get within 30 feet before they notice.
I'd attach the spreadsheet, but there doesn't appear to be options for that.....
Lemme know if someone wants it emailed.
pvandyck