In either case, simply keeping the last known signal lock may very well be one of the worst things you could do.
Depends on what you want to do with the location info. If you want to immediately dispatch a vehicle, then you have to realize that you may be sending one to the edge of the cell tower's range when the caller is many miles away. Or that you might be sending one to the east side of the downtown highrise district when the caller has moved on to the west side of the downtown highrise district. On the other hand, maybe all you want to do is to route the call to the right E911 center. In that case, as long as you are in the right county you are probably OK. In any case, no solution to E911 and VoIP is likely to meet 100% of its requirements, but if you can improve the situation significantly, then it is still worth doing. --Michael Dillon