ruleslawyer said:
Keep in mind that there are several encounters of high CR (like, say, ancient red dragons!) that low-level PCs are incapable of effectively evading, let alone overcoming. Unless you make up a really good reason for that dragon not to just snack on the PCs, he will. He can see further, move faster, and bring them down in a round. I think that's what Ipsissimus means by "running with scissors."
The last true dragon wandering encounter my group had was at night, during one PC's watch. The PC saw the outline of the dragon against the moon as it was en route to somewhere else.
As others have noted, an wandering encounter does not have to be a meeting with the beast itself. It can well be a meeting with signs that the beast is around. Giant owl pellets, the remains of a half-eaten elephant, tracks, and a wierd howl or the scent of something strange that makes the horses shy away are all perfectly good "encounters".
Even meeting with monsters does not necessarily mean a fight. The orcs might decide that the parties are about evenly matched and try to bluff a toll. The dragon might be seeking information about something and stop to demand it. The lions might have just fed (please notice the carcass lying nearby) and be unwilling to do more than watch and give warning growls unless the players provoke them.
Wandering encounters can allow the players to learn something about the world; they do not all have to be combats.
EDITS: (1) I will say this, though, some "Wandering Encounters" should be planned encounters. If you are going to have a dragon actually interact with low-level PCs, you shouldn't be hoping to come up with something off the top of your head. You should know
exactly what that dragon wants, and why.
(2) I do agree with the general idea that most encounters are with low-power creatures, animals, and the like because otherwise there would be no commoners. Even high-level characters should have the opportunity of dispatching these encounters from time to time, just so that they can revel in their power.