I use the Yakima Gunwale Brackets and straps. However the Thule 579xt is essentially the same thing. They will work great for you. No side shifting going down the road, at 70mph they are very secure.
I used to rope off the front and rear. However it is nor neccessary if you space the racks both in front of and behind the center curve of your canoe so it will not slide forward or back, the mounts and straps will hold it nicely without the front or rear rope, even on a bumpy rutted dirt road. That being said, it is always prudent to error on the side of safety. So, rope away my friend.
P.S. these will carry ladders and lumber great also..