The second pillar of thorough oral hygiene.
Flossing or some form of cleaning between the teeth is essential to thoroughly clean the teeth because the toothbrush will not reach this area regardless of the shape or if it is electric (in spite of what the manufacturers will tell you).
It is a skill which is more challenging than brushing and for many of us it is not a habit like brushing (our parents didn’t know about its value, so we weren’t taught to do it the way we were with brushing). It doesn’t matter when you do it, morning or evening, just as long as it is done at least once in a 24 hour period and it doesn’t matter which you do first, brushing or flossing, they clean different surfaces.
The main thing is to make it a habit like you do with brushing. Feel free to feel guilty if you don’t!
If you find the skill particularly difficult you can buy floss that is mounted on a pick, or you can use tiny specially designed brushes provided that there is enough space for them to fit between the teeth without hurting yourself.
Remember that “hard” brushing and flossing does not do a better job, it’s about slow and careful with proper technique but not hard or abrasive. Also, remember, what you are trying to remove is so small you can’t see the bacteria without a microscope. You just have to remove them before they have a chance to harden onto the teeth.
Recently, some articles appeared in the news which questioned the need for flossing. This idea was promulgated by the Food and Drug Administration. They maintained that they could not promote something that was not supported by research. Apparently, their employees didn’t look very hard. There are studies. True, there are not many of them, and they are old but it is not necessary to keep reinventing the wheel, and there is not a lot of incentive for the manufacturers to sponsor this kind of research (nobody makes a lot of money selling floss).
Suffice it to say that you wouldn’t stop brushing and you shouldn’t stop flossing as well.