John, Geometry set up is basically wheel alignment, caster, and camber setting. I have mine set to "Prodrive" setting which basically gives negative camber (top of wheel sits further into the arch than the bottom for want of a better description) gives better cornering as when you turn in the car rolls onto the tyre surface. This setting does however give uneven tyre wear with the inside of the tyre tread wearing slightly quicker but is great if your a fast twisty Rd driver[]
FYI -The standard Subaru setting has positive camber which gives wear on the outside of tyre surface.
A more appropriate setting may be "Fast Rd" but the guy doing it can advise to your style of driving.
"Is it essential or desirable" If your getting suspension work done the geometry will be affected so I'd say "essential" IMHO.
Regarding Teins and warranty it largely depends on which dealership you deal with. I had a warranty claim recently and no questions were asked.[]
Gus