exactly right, sno4rent - I have a spare tire rack, but a few friends have roof racks and have made the 'garage entry' mistake and had to completely replace their bikes. Also...one of them told me that she didn't realize until she had a roof rack that she had to also be careful about drive-throughs - banks, fast food places, etc. and even some public parking garages don't have the clearance. So, I guess that deciding on some memory trick with your garage door opener, if you use one, isn't foolproof.
My friend had her bike on her roof rack and my bike in the back of her Outback and we were visiting a different company office in another city - one that I had frequented regularly but either in a rental car or with my rack on back. She was driving but I was the one who screamed for her to stop, just as we were pulling into the driveway of the garage, when I remembered her bike on top. She said that she completely forgot about it...whew!