Well, I can't say that never giving the players an encounter that they can't beat through combat is ideal. I understand the reasoning behind such a stance, but at the same time I want my players to a) use their heads when they approach a battle and b) realize they do not exist in a vacuum in the world. Now, hints can and should be dropped to spur the PC's towards the right decision. For example, let us say my group fought a dragon a few seesions back, and it came close to killing several party members. Now, travelling through the mountains, they spot another dragon winging it high overhead, and estimate it to be at least twice the size of the one they face earlier. Any reasonable person will be able to come to the conclusion that if something half the size of what we are looking at almost wiped us out, then the thing flying overhead (larger, presumably stronger) should be avoided. Likewise, a group of low level PC's who have just had a tussle with goblins, and are looking for a place to rest, should be going into high alert and be hiding if they find tracks from a band of marauding ogres ("Well, something came through here recently, a group of something actually. Based on the fact that its foot is twice the size of yours and it has sunk significantly into the ground, you'd guess that whomever it was was quite a bit larger than you.")
Even when the clues are not immediately obvious, PCs should show caution. Are those tracks above ogres? giants? what type of giant? This is where those fun skills like wilderness lore (track), hide, and move silently become important. If the parties rouge comes back and tells the party that what they thought were orges (something they have fought and could probably handle) are actually much larger humanoids that are currently playing catch with small boulders, then I would hope my players are smart enough to proceed VERY cautiously (leaving theselves an out if they decide to attack) or even retreat.
Now, say using a 20th level kobold fighter against a group of low level characters I would vehemently argue against. Unless they see some fancy swordwork indicating his prowess, there is no way for either the players or PCs to guage the power of such an enemy, and I would deem this a broken encounter.