I used waxoyl on my jazz, before winter, late last year. Primarily, it was to coat my tow bar and surrounding metalwork - just for a bit of extra security; given how heavily they salt the roads near me.
I think the underside of the jazz is remarkably well made anyway and there really is no need to fully waxoyl the lot. The lack of rusting in original 2002 models is a great example of this. I suppose no harm to add an application of waxoyl, so long as you make sure the material is scrupulously clean.