It's obvious that the use of metal shavings to visualize magnetic field lines was instrumental in Faraday's work to develop electromagnetic theory. But they also did too good a job of convincing people that magnetic lines were real. It is still one of the main arguments of ether theorists. So how much does internal field in the shavings and their dipole interaction affect the pattern they show? Maybe they do not show real magnetic field?
Yes, since the metal shavings would act as small dipoles the pattern would indeed vary from real field lines.
But as we try the experiment with finer and finer particles, we would get closer results as the finer particles would hardly affect the field and their small inertia ensures that the stick to the real lines.
Check this out https://en.m.wikipedia.org/wiki/Ferrofluid Using fluids, we get much better picture if the field lines.
However the lines are quite similar in both cases, so we can assume the initial experiments give us a picture that's quite close to the real one.