...and most people would be:
a. Incorrect regarding relaxed standing posture
b. Correct regarding activities where the trunk is bent forward
c. Correct regarding activities where one leg is in front of the other (split stance, lunge, etc.)
The short answer is: It depends...
Here's the thing...
In standing it is ideal to have a posture that is relaxed and responsive to your environment. If you gluteals are contracted in standing all the time this will simply not be the case. I like to bring this up because many experts give this advice (I just watched a youtube video from a 'posture expert' who is recommending tight 'bum' muscles in standing) and based on her advice, I should never let them go...? what about when...
anyway, getting back to my point:
1. In normal standing, learn to control your pelvic and lower back region with good deep abdominal control: learn how to do a pelvic tilt without using your butt.
2. Learn to activate/tighten your gluteals in split stance, 1/2 kneeling or lunging positions (basically when one leg is in front of the other). Add resistance with bands, cables, dumbbells, etc.
3. Make sure you do a lot of training with a neutral pelvis in functional positions like squat, single leg squat, hip hinging, etc, to train better posture and lumbopelvic control.
If you have difficulty maintaining a neutral spine with these type of activities, try using the POSTURE BELT for Back Pain from ActiveOrtho. This product was made to treat back pain and prevent back injuries by correcting posture and improving spine stability.