You can ballast track (or not) any time after it is laid down where you want it, and you've done test runs to make sure there are no defects in your track work. There is no fixed order in which things have to be done, and anyone who says otherwise isn't doing you any favors.So can I ballast now? Or should I wait. Im not planning on plaster mountains (did that with my son 15 years ago, it was fine but never felt solid to me). Foam carved. So at least it won't be plaster itself getting on the tracks. Something else.
Personally, I regard ballasting track as a necessary evil. I will usually ballast about 10 feet of track, then go work on something that I actually enjoy doing for a while.