Hiya!
It depends entirely on if the DM is a "new skool" DM, or an "old skool" one...or how much they lean towards one side or the other.
As an Old Skool DM, I often don't have a "choice" to set things up a certain way for the PC's. Well, at least not if I want my world to be even
remotely believable and sustainable over the course of years if not decades. I can't just "decide" that some hills have kobolds when the PC's are level 2, then they have orcs when everyone is level 4, then they have ogres when everyone's level 6, then hill giants when they hit level 10. Then the players decide to make new characters for some reason and head into the hills...and suddenly they're back to 'only' encountering kobolds again.
If the Giant Hills are ventured into...you'll find Hill Giants. I don't care, as a DM, what level the PC's are. My world dictates that "The Giant hills are known for being over-run with ogres, hill giants, cave bears and even a few stone giants. Travelers are to be wary when they risk taking the Stone Road through to the other side"...well then, that's that. Ogres, giants, cave bears will be 'common' in as much as any of them can be. As a DM, my hands are virtually tied. And, IMNSHO, any DM that 'downgrades' or 'upgrades' creatures in some area/adventure to be compatible (re: "balanced") to the players PC's is doing a serious disservice to both his players and himself. But then again...I'm old and crotchety like that!
^_^
Paul L. Ming