Modern roleplaying games have a surfeit of rules for balancing encounters. It wasn't until recently that I realized these rules are near-useless. After years of planning encounters following these guidelines I can say that they are seldom accurate. Two monsters of equal challenge rarely pose the same threat. That's all beside the point, really. The question I want to ask is: as a DM, do you scale your encounters to your party level?
That's the reason I force my players deep into the wilderness if they want really tough fights: powerful monsters simply don't lurk within the bounds of civilized borders. In the few cases that they do wander within the bounds of a kingdom, it is usually to challenge that authority.
That's the long view of things. Within an adventure, however, the answer becomes a little more complex. Each adventure should, in my opinion, have several encounters below the party's level. It should have a fair number that are adequate challenges and even a couple that are very, very hard.
All of this is on mind because one of my groups is getting ready to venture deep beyond the bounds of civilization and they're going to see the difficulty of encounters spike almost immediately. I hope they have the good sense when to hold their ground and when to run.