Hi Kevin,
It really depends on what your shooting, i started with an 18mm-70mm then got a 70mm-300mm then 50mm prime,then a 10mm-20mm (which i love by the way) then the big one 80mm-400mm OS.
I have recently sold the 50mm and the 70mm-300mm (which i really miss for the weight and size for the zoom you get),and bought a 18mm-200mm which is great to leave on the camera.
Much of the time i find it's not the case of do i need it? it's more of a case of do i want it? i usually last about a week cave in and get one then bang in the overtime later...
My next one will be a 24mm-70mm 2.8 which i want as i am trying to get into shooting models.
well enough of me spouting hope this helps (maybe just a bit???)...
Regards Steve.