I never worked for GM, and thus didn't have any special deals getting new cars (other than getting quicker good pricing by working for a supplier). I've bought probably 20 new cars which included Chevrolets, Plymouth/Chryslers, Pontiacs.. etc. I special ordered 4 of them, because I planned to keep the cars for a long while, and wanted them a specific way that the dealers didn't typically handle (as William, JohnZ points out).
Based on my experience, and from many other cars purchased by friends and family, I agree with JohnZ's assessment that probably 80-90% are purchased off the lots, and 10-20% ordered as 'sold customer cars'. In fact, from all my friends and family who have purchased new cars over the last 45 years, I only know of one friend who purchased a car for order. I'd venture to say that MOST of the 10-20% of 'ordered' cars are by people who have special relationships with the manufacturer's (ie. employees who get special deals).
I typically go to a dealership for a new car with a list of options/colors/interiors that I want (from having perused the literature and the lots, making notes off the window stickers, etc).. so I know what I want. I seldom trade in a car when I buy a new one.. so that did not influence me at all (as it does most buyers.. they buy where/when they think they are getting a good price for their used car - but that's an entirely different story!)..
Everytime I've ever gone in and buy and given a dealer my list of colors/options etc.. they *always* say.. We don't have one like that, but we can probably FIND you one from another dealer within a day or two... sometimes that is true.. sometimes it isn't. If I want those options badly enough in those last cases, I order it.
Generally speaking a dealer will do just about anything to sell you a car from their inventory (or another dealer's inventory - because they trade one of theirs for it) rather than order it for you. When I've ordered a new car, I've had to tell the dealers that if they won't order it for me, I will find another dealer who will.. only THEN have they been willing to order it..