Depends entirely on the situation:
If it's a scramble mission I just charge in because that's all you really can do. You take off and attack and hopefully land correct side up later.
That aside, I'm a rationalist--
my plane, my ammo/fuel/damage, our numbers, our altitude
weighed against
their planes, their numbers, their altitude
AI combat:
The plane type is probably the biggest factor I consider (along with whether I have ammo/damage to my plane). I'm willing to take on more Eindeckers with fewer Nieuports, for example. But if it's Nieuports vs Albatross aircraft, then I would avoid getting involved when out numbered. It's a little checklist I do before engaging, if I have the option. In some situations the choice is made for you because they're close and above. In those situations you just prepare for the attack, or dive for cover if you have friendly lines or help nearby.