When I was younger, I followed my routine to the letter. It was something I could have faith in: just follow the routine mindlessly and you'll have a good workout.
While this approach still seems ideal to me, I've recently found I feel a lot better about my workouts and have less pain if I actually focus on what I'm doing and adjust every rep to maintain muscle sensation.
In fact, I've seemed to notice it's hard to focus at the beginning and end of a workout. At the beginning, I warm up more, until I can feel the muscle and focus on it. Once I start finding it harder to focus, I start wrapping the workout up, even if I had planned for more.
Tl;Dr: do you think its important to focus on your workout and adjust by feel, or can you follow a routine to the letter and let your mind wander?