From my experience, players like consistency. Also, they like it when there is a difference when their PCs try to accomplish a given task in different levels.
I know that as a GM I can decide that *right now* the PCs can avoid being spotted by the Hidra on a 'Very Hard' manuever, but then I need to remember it for the next time they try to do the same, and adjust for their level etc.
IMHO, If the PCs try to accomplish something and their opponent (monster/NPC) has a say about it (like sneaking on the Hidra), I need to let them roll and adjust the opponent roll accordingly. Its more 'realistic', in the sense that most of the time - you don't know how well you did.
I disagree regarding character knowledge and player knowledge. Most players can't handle this very good, and even if they do, its forced and unnatural. I prefer to let them decide with a clear head, and not effect their decision with 'real life' information.
Note that this is not valid for jumping over a pit - this is easy enough for the PCs to judge their ability to do so.
All I want is a quick method of determining the ability of a given opponent to do certain things. I agree with Arioch that modifing the PCs roll is the quickest way, but I think that method relies too much on the PCs roll and gives away too much information.
If you let the opponent roll as well, the PCs might roll poorly and still succeed in their attempt (The PCs rolled low in their sneak attempt, I give the Hidra an 'Easy' awareness manuever to spot them and it fails - the PCs did a terrible job sneaking, but the Hidra did a worst job in spotting them).
Wow that was long...
What do you think?