I think brushing first would help to loosen particles for the floss, hence why your dentist may say that! Personally, I brush to loosen everything up, then floss between, then use a vigorous salt water or mouthwash rinse to get everything on up out of there. Really, though, I think as long as you both brush and floss you are good! 🙂
I floss then brush and night. I brush first in the morning though, then after I’ve had breakfast I floss. Hope that helps.