i have an Evo--with a 4g toggle widget for Wimax
(yes, Wimax<LTE, but its pertinent in this discussion)
there's 2 things to consider here
1) did VZW end up putting a LTE toggle on the final phone?
yes? you can keep LTE off to conserve battery & only use it when you want. it won't turn itself on when you're inside of an LTE area. it'd require a manual push of an on-screen button to allow it to search for LTE service and would then receive the LTE signal.
no? your phone will constantly switch from 3G to LTE depending on the signal it receives. if LTE signal is too poor, your phone will switch to 3G
2) 4G is a signal, like 3G, in that it's a dedicated bandwidth specifically for Verizon (in regards to LTE 4G). even if you're in the area, shown on a 4g LTE map, you might not have as strong of a signal in other parts of the 'covered area' map.