My understanding is that if you train to failure, it should be on your last set. Also, the volume should be kept low. Pretty standard HIT stuff. Some big names made great gains this way. This has been working for me lately. I will do this until it stops working, then change it up...again! For example, when doing chest I first do 40 pushups to get the blood going, two warmup sets on incline bench with light but progressively heavier weight, then 1 working set to failure +1 (spotter required!). I define failure as the last rep I can get by myself with good form. Further chest exercises require no warmup sets: 1 set of incline flys to failure, 1 set decline bench to failure, and chest is done. I like to change the rep scheme from workout to workout, so 1 week failure is at 12ish reps, next week at 8-10 reps, next week at 6ish, next week back to 12.
This has helped me break through a plateau and start progressing again. Everybody is different and what works for some will not work so well for others, and as well as this is working for me right now, I know in time I will need to do something different. But I don't like to change just for the sake of change...if it works for you, do it until it stops working, THEN change it up.