Andy,
Aluminium profile is very popular and based on the pictures we see on the forum is straight enough for directly mounting the rails. Problem is it costs an arm and a leg in Australia. My own machine has the rails mounted directly onto the steel section and relies on the straightness of the steel. It works OK for fully suported round rails but that may well not be good enough for Hi-Win or similar. The alignment of the rails to each other is done as pippin88 describes. Whether that's straight enough for you depends on what you want to make with it.

I'm not sure if anyone has tried a hybrid design with a steel frame and aluminium for supporting the rails. We're then into possible problems with differential expansion!

Kit