A lot of people might disagree, but here is a thumb-rule I apply to things like this. Don't feel guilty. If their content is not interesting and will end up trashing your feed, don't follow back.
It depends on your decision, If you would like to follow them then you can otherwise it's not needed.
If the person that followed you has something interesting to share, then you should follow them back. This might in future turns out to be a fruitful connection and can help you out in various ways. It is always appreciated if you connect someone back.
It depends. I believe that those who you follow are your interests, so you should follow only interesting content for you, or your business if it is biz account.
Just for gaining traffic many people follow their followers back, but it is not a right way. It will surely crowd your website with follower but it does not imply that you will get high ranking for your website. Real followers are best for increasing the traffic as well as the reputation of the website. If you feel that the people who followed you will be beneficial for you in future then only you must follow them back.