It's very rare to floss.
For a lot of dogs, there would be risk involved because it's string in the mouth. The most important thing to remember is that you have to be extremely careful that it isn't swallowed.
I have used the regular dental floss (long string in a plastic box) before. She doesn't like when I go in between each tooth, but she likes when I hold both ends and she runs it through her own teeth. lol. I have also used the individual flossers with the plastic handles (like 100 to a bag). Hands don't get as slimy that way and there is less risk of them swallowing some, but be careful not to hurt the gums with the plastic edges on those things.
She pretty much hates it unless she gets to play with it, but she isn't given a choice.