Anonymous feature was created for a reason. Some people here unfortunately tend to abuse this feature. So these reasons below are the reasons to use/not use this feature:
You should use Anonymous feature when:
1) If It’s a “guess who?” question
Otherwise it wouldn't be a "guess who?" question.
2) If you want to share a personal experience that you would not share with your username
I understand that you want to go Anonymous in this situation
3) If you want to post on a sexual question in order to avoid creeps
Especially if you are a girl. You have to deal with a number of creeps.
4) If you want to post your pic on “How Do I Look?” topic
If you want to post a body pic of yours for example,i undestand you wouldn't do it easily with your nickname.
5) If you want to post something on a question of a user that has blocked you or you have blocked
Without insulting him/her of course, or if you were blocked don't ask why you've been blocked nor mention who you are. If you want to share an opinion on his/her question politely, go Anonymous and do it.
You shouldn’t use Anonymous feature when:
1) If it’s a question like “What’s your favorite food/drink?”
I don't understand why you should go Anonymous here
2) If you want to ask questions like “Girls do you want to see my penis?”/”Girls message me”
3) If you want to insult another GAGer
4) If you want to bash some particular group
2,3 and 4 shouldn't be done neither anonymously nor with your username of course.
5) And most of all don't use anonymous feature 99.9% of the times
Unfortunately there are some users who go Anonymous always and post with their username very rarely. I disagree with this. You shouldn't go Anonymous without a reason. You might post very good opinions and someone might want to follow you.