It's in my opinion the mixture that makes the difference: Should you always train to failure? Probably not. Should you never train to failure? Probably neither as well. Change is what keeps your body engaged.
This article nicely sums it up:
From Willardson’s (2007) review of the research on this topic, he encourages not always training to failure as this may be a contributing cause to overtraining and overuse injuries. He also cites three studies that show there may be a decrease in growth-promoting hormones if training to failure is done continually. Depending on the client, he suggests to alternate going to failure in sets on sequential workouts, or even alternate weeks. Willardson expands that training to failure should be varied, just as all acute variables of resistance training (e.g., numbers of reps, number of sets, rest between sets, order of exercises, choice of exercises, etc.) are varied in periodization programs. More importantly, Willardson suggests that the athlete or client should stop the set when the exercise performance technique is being compromised (e.g., poor posture, body shifting, accessory movements, etc.) to lift the weight.
http://www.unm.edu/~lkravitz/Article%20folder/trainfailure.html