Bedroom registers

Should I use a round or square registers in bedroom ceilings? We just had a heating system installed in a cottage we are renovating. The first contractor ran a trunk line in the attic with a round flexible duct to each of the bedrooms. In the bedrooms the duct passes through a metal plate with a round hole. It would then use a round register. We had to fire the contractor due to some quality issues. The person we hired to fix the job suggested we put an 8x8 box with a square register in each of the rooms. Which type of register would work best for a bedroom? Would one allow the better adjustment? Does it matter?