Hi Andy,
That's a great question. I would keep the main categories as close as possible to the root domain.
My silo structure would look something like this:
webiste.com/washing-machines - Main category
webiste.com/washing-machines/safety-issues - Subcategory
webiste.com/washing-machines/reviews - Subcategory
webiste.com/washing-machines/spares -Subcategory
webiste.com/washing-machines/buying - Subcategory
Ross