Training to-failure has it's good points and bad points. To keep it simple; training to-failure can be really tough on the CNS and isn't really required to make good gains.
But training to-failure once in a while can be a good thing because it tells you where you are at in your progress and can give you a real big growth response. But, staying one or two reps short of failure seems to be a good way to do things a majority of the time.
There's a saying; Training to failure teaches your body to fail sooner.
There is a little truth to that saying. Your body is highly adaptive; if you constantly apply a stimulus like to-failure training to it, it will find ways to combat it. At first it will get stronger, but if it can't keep up it will start shutting things down early to avoid the trauma.