Formal nights are a very "sticky" issue. Some people love them and others (like you and me, who have to dress up all week, hate them).
Just remember that in addition to the picture taking around the ship, there are also dining room table pictures taken on formal nights. If you want to be included in a "table shot" to remember your dining partners, in good conscience, you should attend the dining room on formal nights. If, you don't care about your fellow diners and don't want to be in the table picture, then of course, head to the lido buffet.
It is YOUR vacation. You worked hard all year to earn the money to pay for this vacation. #1 HAVE FUN!!