Absolutely. Think of it this way... you are used to your child's behavior, if he throws a fit, falls on the floor, grabs toys from other kids, etc. etc. the way my son does sometimes when he is overwhelmed... to you it is just another day, but to people who don't know your child, who are on vacation, it is a child out of control with parents who are not using good discipline. If there is a "touchy" line, and you can avoid having your child melt down by using an alternate entrance, you are doing everyone a favor, especially your child. You and your child are both sensitive to how other people react to your child's behavior... if somebody else gets upset or worked up, it can send an autistic child over the emotional edge.
Another consideration... the GAC isn't just about the queues. It is a short-hand signal to CMs that you may need ASSISTANCE. If you need to get to a quiet place, need alternate seating away from a crowd, need to leave quickly, it is really helpful to be able to flash the GAC -- the CMs get it right away, you aren't just another parent with a cranky kid, your child has special needs and you need a break. If you are having to restrain your child, and people are making comments or giving you The Eye, most intelligent and sensitive CMs will cover for you, cool down intrusive guests, really smooth things over. I have had this happen, many, many, many times... many CMs are wonderful about helping calm things down. Not all, but many.
Click on the link in my signature for information about the GAC, and some tips about traveling with sensitive kids.