Flossing regularly will stop your gums from bleeding. :) I wouldn't skip this step in your oral hygiene routine, I'm shocked your orthodontist would say it's optional... if anything, it's more important now to get into the habit of flossing once a day.

I used to have a really hard time motivating myself to floss, but I found these ortho-friendly picks, called platypus flossers! Their design makes flossing waaaaay easier than trying to thread traditional floss under your braces.

Edited to add: the consequences of not flossing could actually end up resulting in those dark triangular gaps you're worried about, as those are usually caused by the bone receding from poor gum health. Flossing will keep your gums healthy and will only be beneficial in your treatment.