hey,
when rolling without using your hands, its best not to try to roll directly down the line of your spine....
If you can, try to learn to Roll first on one shoulder then to the opposite hip.
If you do this, you will be putting less strain on your neck.
I've got several years of experience teaching gymnastics floor work, where the emphasis is on symetry, ie you roll down the line of your spine. I've also got several years experience teaching martial arts, and while it isnt judo, we have a judo component in all our gradings. We teach Rolling from shoulder to opposite hip, and I find that when attempting a roll with a weapon (or poi in this case) it is much less of a strain to do it from shoulder to hip.
Your milage may vary.
With the butterfly rollout move tho, it may be neccessary for you to have a symetrical rollout, and so all I can say is, put your legs waaay apart and lean over as close as you can to the ground before rolling, thereby reducing the impact / risk of strain.
Good luck.
Josh