Do all bolts need washers? Usually, only two factors are involved. Is the hole oversized, and is the bolt configured to work without the washer?
Turn the bolt over and look at the underside.
The first one on the left is simply flat. Number two is an AN bolt and has an area that acts as a washer. The third one has an enlarged surface, and the fourth has a separate, permanently attached washer. Only the first one must have a washer. The second one, does not require one, the third one has a washer built in, and that last one has a permanently attached washer.
Only the second one should be used on aircraft, though, as it’s the only one whose manufacture is fully pedigreed.
AC 43-13 section 7 allows you to skip the use of washer under an AN bolt. But a few AD's do, at times, require washers under both the bolt and the nut.
If you’d like to read further on this topic and don’t have a copy of AC 43-13 beside your bed, you can download it HERE.