In one word ... NO, absolutely not! The only reason I would suggest doing this is if it's something you would like to do! I always tell my brides to go with the colours and flowers that they WANT, and that they LOVE. Everyone has their own opinions on what looks good, the important thing is that it's what YOU want. If you want to take into consideration the colour of your bridemaids dresses when choosing flowers, that it totally OK, but if you are up for just choosing flowers and their colours based on what you like, then do that.