Whether or not to train to muscle failure is a hot topic on which even the most learned coaches do not always agree. Nevertheless, from theory and practice we can distill a number of situations in which it is better not to train to muscle failure.
1. IF YOU DO A LOT OF VOLUME
The relative training intensity (how far you train to muscle failure) and the training volume (how many sets you do per muscle group) are inextricably related: more of one means less of the other. If you train often (3 or more times a week) and you probably do a lot of volume (10 to 20 sets per week), you should preferably train your sets slightly away from muscle failure (one or two repetitions, or 1-2 Reps In Reserve , RIR).
Remember that training to complete muscle failure only provides a tiny bit more stimulus than when you train with RIR. At the same time, training to muscle failure brings disproportionate fatigue, which hinders your muscle growth even when you do many sets. By staying slightly away from muscle failure, you can on balance do more volume (and therefore more effective repetitions) and therefore achieve more muscle growth.
If you train a muscle group no more than once or twice a week with a maximum of 10 sets, you can safely train to muscle failure more often, for example with every last set of an exercise, or with all sets if it is an isolation exercise. An example of such a training program is High Intensity Training).
2. AT THE BEGINNING OF THE TRAINING
If you train for about an hour, it is good to save your strength and not to train all sets to the limit. A logical structure of a training is first to do compound exercises with 1-3 RIR (see next point) and towards the end some isolation exercises to failure.
3. DURING (HEAVY) COMPOUND EXERCISES
It is better not to train heavy presses, rows, deadlifts and squats to complete muscle failure (if that is even possible). Due to the heavy weight of these exercises (6-10 repetitions), you get enough stimulation at 1-3 RIR.
In addition, staying away from muscle failure is safer: you prevent injuries.
You can also do compounds with light weights, but then a lot of unwanted fatigue accumulates, other than local fatigue (the fatigue in the muscle). Doing very long sets is simply not an efficient nor a pleasant way of training that causes a lot of central fatigue, meaning you have to stop the set well before muscle failure. It also takes more time.
4. EARLY IN A TRAINING CYCLE
Do you train in cycles, or periods of 5-8 weeks of training followed by a deload? Then you can choose to slowly increase the relative training intensity. For example, 3-4 RIR in week 1, increasing to 0-1 RIR in the last week. The deload then gives you enough time to recover from that tough last week. This way you create progressive overload in your relative training intensity.