Andy, if you are going to use bunks (and I think they are a good idea), rather than having keel rollers, a couple of rollers in a V formation about inline with the trailer wheels will both support the boat as it is pulled in and act a centralising guides. Another set nearer the bow will guide, support and assist launching. If you imagine the stern of the boat beginning to float at launch it will become free of the bunks, the front rollers will take the bow weight and allow you to roll the boat off easily. You do have to ensure the bow snubber, if you use one, doesn't interfere with this. When you are winching the boat onto the trailer, it will catch onto the mid rollers and the tension of the winch line will pull is straight and help it align as it is winched in. Then you just need a protection roller on the centre of the rear cross member to prevent damage should the boat contact it.