When I had carpet put in at our old house in the basement, the installer told me to put the baseboard trim last. That way it will always be nice and snug on the top of the carpet with no gaps. I am looking at the carpet in our new house and I would say that looking at the carpet already installed that the trim could have been put in first. I think that it really comes down to the preference of the carpet installer more than a right or wrong way to do it. I think that if there is no trim in place when they lay the carpet, the installer can get the tack strips a little closer to the wall and really get the carpet in there easier than if there was baseboard in the way, but I also agree that a really good installer should be able to do a great job either way.
I think that you are right on track for getting the baseboard in since you are doing it first. Leaving that small gap allows the carpet to be under the trim in the end. It is also a lot easier, I would think, to make sure that you get a nice straight install since you're putting the scrap trim on the flat cement and installing the good trim on that nice even surface.
Let me know how it works out. I will be doing the same thing in the spring....
