Axle slip sucks. If you don't have a skate tool on you, and this happens, you can temporarily fix it by banging it back into place. Fair warning, though, this just makes it worse in the long run. Don't let anyone tell you how to ride your trucks, in fact, don't even listen to them. Find out what is comfortable for you.
Some people like really tight trucks, some people like loose trucks.
A popular style is to keep the back truck slightly tighter, for ollie stability, but it's all up to you. And that's the beauty of it all. Of course, one of the biggest factors in determining which trucks to get is the weight. Some companies have created ways to lighten the weight of the truck, while keeping the durability. Grind Kings and Thunders are on the lighter side, while Independent and Destroyer are heavier.