I'd say that it's better for you to do so.
My reading of the documentation at https://experienceleague.adobe.com/docs/id-service/using/id-service-api/configurations/overwrite-vis... is that it's for the use-case where the app causes the standalone web browser to open. In that case, you'd want to keep the MCID consistent with the one used by your app, so that you can report on the user's journey consistently.
My understanding of WebViews is that their storage is separate from the storage used with the standalone web browsers. This is particularly so with iOS, e.g. a cookie that is written in an app's WebView can't be read in Safari, and vice versa. Keeping in mind that the user could also open the WebView's page in their standalone web browser (e.g. via the WebView's share button), I think it makes sense to overwrite the MCID (and AID) so that your user journey reporting doesn't break in that scenario.
