Can it really be said that something as large as a galaxy can have a "shape", which the positions of its constituent stars are their positions at various times in the past? A star 100K ly from us is no longer in the position we see it, same for stars 50K ly away and so on. Thus, a "shape" is not sufficiently deterministic and is only phenomena from a particular perspective; the shape doesn't exist.
Thus, the wave density models, which assume a universal "now", would be inaccurate. I know these time differentials must be considered, I can't image they're ignored, but it's never mentioned if it's just insignificant or built into the model. I'm sure this is a naive question, but I never come across anything that addresses it.