Brian, I always carry mine in the bag even during competition. Usually use them on the range before the round anyways so they are almost always in my bag.
DOG, I started out at about 6 inches apart and as I progressed I put them closer and closer until they are just wide enough to get a ball through. It's a good drill to practice.
Golfchick, I don't use them to check my swing plane, just alignment and making sure the ball is going where I want it to.