The way I see it, there's three ways to handle combat in games.
1) All monsters exist where they do based on the game world. The players hear about Storm Giants in the mountains near their home. These Giants will be 10HD monsters whether the players encounter them at level 1 or level 15.
2) All monsters CAN be killed/defeated, but the players might have to work for it.(expend magic items, outsmart the foes, etc) Ideally, there will be more than one possible solution!
3) All monsters can be killed in a straight fight.(also known as the 4th edition)
Please chime in with your preference! It helps me a ton to know these things so I can run better games, as well as not accidentally invite someone to something he or she wouldn't enjoy. I've run all three and had fun with 'em!