“Do I need to floss if I use Invisalign?”

Yes especially since retainers can prevent saliva from flowing in between teeth.

Yes you need to floss. The dental plaque that you remove while flossing is still present(even more abundant) while you are wearing the Invisalign aligners.

Not sure why you think having Invisalign on would change anything? As always, flossing is only for those teeth you want to keep. Clear aligners significantly reduce the natural cleansing action of saliva and trap any cavity promoting stuff on your tooth/teeth making it even more important you get all that off before putting in the aligners. You could talk to your dentist about something to put on your teeth or in your aligners to help strengthen your teeth.