So what levels are encounters in the wilderness? Really...how tough a hexcrawl or more open wilderness is matters a lot on expectations set up when initially populated. The OD&D wilderness charts were full of party-killers unless folks were name level and/or very large groups as was the Expert booklet version of the game. AD&D was a little softer on folks as it used the monster frequency to some extent, walking from town to town was still pretty dangerous.
An OD&D scale of danger looks like the average wilderness journeyer was about 9th level so there we are level 9 wilderness. This creates issues with expectations of the game but it also explains why dungeons are still out there all the high-level characters are constantly busy clearing the area around their castles but also reduced the plausible likely-hood of going to the market and buying anything.
Let's say you have a half dozen cows you'd like to take to the nearest town to sell, what's the chance you'll survive and get the cows there? If you have a 1 in 6 chance of a risky encounter traveling to a neighboring town and there is a 1 in 12 chance of a red dragon... that's a 1.39% chance of bar-b-q cows., not to mention how likely wolves, bears, orcs, bandits, and trolls are going to eat the cows instead. Maybe the chance of death and destruction on travels sets the crazy D&D prices.
For me the general level of a wilderness/outdoor area is sort fo a solution or at least necessary figure to keep in mind. If a wilderness is setup to be 6th level more then 50% of encounters would be with 6th level/HD or less creatures.
2d6 encounter table for level 6 wilderness
2. 1 HD
3. 2 HD
4. 3 HD
5. 4 HD
6. 5 HD
8. 7 HD
9. 8 HD
10. 9 HD
11. 10 HD
12. 11 HD or more
Not exciting but it gets the point, Most likely bumping into a small group of trolls in a level 6 wilderness, not too many wimpy creatures. This isn't a very naturalistic break down...because it's a game. You could go wider of course and play with the dice and ranges a little more.
2d10 encounter spread for level 6 wilderness
11. 6 HD
Keep in mind this is setting level not player level. If you have a 6th level wilderness a walk in the wilds is full of challenging adventures for a 6th level party, a death trap for 1st level, not a substantial threat for 12 level or higher. In such an environment it's easy to tell who the tough guys are, they go outside of heavily patrolled hexes.