Subject: In rocket launches, when does the delta V from Earth's rotation kick in?

Text books dealing with rocketry commonly discuss the delta V assist from 
Earth's rotational velocity, and conclude: "The best place to build a 
launch pad for hurling satellites into orbits is on the equator, and the 
best launch direction is always due east."  With the tracking resources 
available to and through NASA I am assuming that this delta V idea is 
scientifically proven every time a rocket goes up (and a shuttle comes 
back down).  If so, then I am hoping that someone could tell me exactly 
when a rocket experiences this delta V.  I am assuming that it would be 
either:  1) before launch 2)at the moment of lift-off, 3)at the moment 
orbit velocity is attained, or 4)somewhere between launch and orbit.

Re: In rocket launches, when does the delta V from Earth's rotation kick in?