The preception is VFR pilots are updating their details so in the event they go missing SAR know where to look.
Might be so, but it is more likely your instructor is using it as an excuse to make you talk to the big scary controller, as there are not many other VFR opportunities to do so.
We have a stupid instruction that we have to inquire if the diversion affects a SARTIME, when really ATC should have nothing to do with the things. If you notify an amended SARTIME we should process it, otherwise complete non-interest.
Whatever you say is fine, provided we can pick out from the noise whether it will affect a subsequent clearance request, not always easy to do.