Personally I try to avoid putting Verdana or Georgia as backup fonts (In practice, the stacks you've listed are fine because almost no one will not have Arial installed, but by that logic you could leave out Verdana altogether). In principle, if we're imagining a situation where Arial isn't an option, falling back to Verdana will usually look pretty crazy, because Verdana is designed to appear so much larger. 12px Verdana just looks a lot bigger than 12px Arial. All this (including the part about it not mattering in practice) goes for Georgia (vs Times etc) as well.