It depends on your personality really and what it is you want to achieve.
If you are new to yoga, then yes, guidance for the correct posture is a start.
But what I always guide my students to do, is feel what the body is telling them. If the body is saying "that bent knee doesn't feel right" or "that staging arm is uncomfortable" then following this intuition is key.
So many of my students want to do exactly what the next person next to them is doing - but how do you not know that the person next to you is not adjusting the pose according to their needs?
Listening to your breath, especially the rhythm of it, is your first clue, then your gut may kick in, then your mind may provide your final prompt.
The key message is listen to your body. If you still not sure then private yoga may just be your answer, away from a group class, to get some extra assurance you are on the right track.
Practicing on your own away from classes, is a great way to cement the posture, and to gain the positive affects of yoga in your daily life, while tuning in to what your breath and body are telling you.
Practice makes perfect, but perfecting something so rigidly can distract you sometimes from what the body is really telling you to do.