^ I think that's unlikely. The question, as I see it, is: "would the party, especially one geared to take advantage of it, benefit from getting the drop on the monsters?" Getting the drop on the monsters tends to allow for, at the very least, a bit of pre-buffing and sometimes a surprise round, although the latter is hard for the entire party b/c it's rare for the entire party to be stealthy.
For any encounter worth its salt, i.e., one that is any challenge at all, stuff like that should really help. And, that's in addition to any kind of "intangibles" like planning tactics, pulling monsters, and so on.
2 rounds of pre-buffing in D&D can be enormously powerful. It can turn a challenging encounter into a trivial one. Just think of how powerful the War Weaver is if reasonably well-optimized. And, these kind of advantages are general ones. Sometimes it won't matter, if you're facing something like a Golem, which are pretty terrible at Spotting but also pretty easy to out-tactic to begin with, then it's a wash. But, that's true with regards to a lot of things.
And, the vast majority of monsters are vulnerable to sneaking. There are exceptions: Vrocks, Succubi, and Beholders, all have huge bonuses to finding people. But, that's what it takes for them even to be competitive. I don't have any hard numbers, mostly just my gameplay experience, but I don't think Spot/Listen scores are particularly high among the monster manuals, and only a few special abilities (also pretty rare) shut scouting down, and even then most of those can be countered by a reasonable optimization or a minor commitment.
I'm just ballparking it, but I think anyone who wants to can comfortably sneak past 80% of the monsters printed in books.
There are two caveats to that. First, it won't always work. I don't think that's a damning fact with any tactic, but some monsters will have great rolls or great scores or some ability like Blindsight. So, you'll need a fallback plan. Second, it requires a DM that isn't going out of their way to constantly thwart you. Again, I think you can say this about anything in D&D. But, if a DM really wanted to stop you from sneaking, he probably can.
It's just that relatively few monsters come pre-equipped with the means to do so, and those monsters are still as threatening as anything else at their CR.
EDIT: it may get a bit harder at higher levels. Monsters HD tend to scale up meteorically, which means their skills can sometimes, too. But, you also have a lot more resources available to you -- it might just require a bigger commitment.