From lingering around other people's games, it has become apparent to me that a lot of times monstrous demihumans are simply aggressors. When they are encountered, they may demand some gold, but more times than not they will attack just for fun. I think that there's something to be said for a good fight now and then, but there's also something to be said about giving your monstrous demihumans (and enemyhumans and regular demihumans) the power of reasoned thought. Most small parties of goblins, for example, should know better than to attack well-armed and well-armored adventurers unless they have a huge advantage in numbers.
Smart foes are the basis of a challenging game, and they also help to bring a sense of realism to the setting. Not every monstrous demihuman is a ravening beast that desires nothing but to bathe in the blood of their foes. Many are quite capable (and should be portrayed as such) of advanced tactical maneuvers, but also in judging the relative strength of their enemies and making a decision based on whether or not they think violence is the best solution. Hell, violence is almost never the best solution. Unless they are particularly bloodthirsty (gnolls might qualify here, and hungry orcs probably can as well) there's no reason to go out of their way to attack someone that they can intimidate instead.
Intimidation is free, there's no chance that one of your own minions is going to die, and it preserves your strength in case a real battle is coming. Hell, goblins, orcs, ogres, and anything else you can communicate with before they try to flay you can probably be bought off if they think you're too tough a nut to crack. Demonstrate some magic, since wizardly prowess will often be unavailable to your monstrous foes. That generally makes them a little more docile!
There's no reason a group of ogres, having encountered an adventuring party of folks armed in clearly magical gear and toting a wizard and a cleric along with them, wouldn't agree to simply charge a fee and leave them be. Of course, at that point they probably smash peasants and local merchants into pulp, so the party might have a vested interest in removing them. However, less justice-minded adventuring parties might decide to try to pay them and keep them on as retainers.
How do you think all those evil NPC organizations get monstrous mercenaries? In pretty much the same way. Now, it can be a challenge to control your monster-men, particularly in societies where they aren't frequently resorted to as hirelings. You might get mean looks, and your ogres might start pounding people just because they want to. You'll have to keep a sharp lookout to prevent situations like that from developing or mitigating the damage when they do. Keep it up long enough, of course, and another adventuring party is going to hear about you and maybe decide that you've done enough damage.
Either way, reasoning creatures that aren't under the spell of some ravening need for slaughter shouldn't attack without parlay. If there's a way to get your enemy to surrender without a fight, that is much preferable for them. Of course, in the case of orcs that means the PCs have just become slaves. In the case of goblins it could mean they're slaves, dinner, or just executed for fun. Gnolls just want to take you home to their camp and gnaw on your guts. Just because the results of a surrender can be as or more gruesome than a battle for the PCs doesn't mean the enemy won't offer it.
Likewise, there are few creatures in this world or any world that will fight until every single member of their group is dead. At best they will try to run, and at worst they may even attempt to appeal to the mercy of the adventurers and surrender to them. Of course, in the 10th Age you can never trust an orc's surrender, but other creatures may be captured as long as one is a careful jailor.
You should think through the motivations of your monstrous NPCs every bit as carefully as you think through those of your elves, dwarves, and men. For those of you that don't play D&D, you probably don't have this problem. There's something about saying "goblin" that seems to trigger an instinctual response in most GMs, one that says "only by fighting all these goblins can the PCs deal with them." But that's not true at all... All kinds of deals can be struck.