Page 81 of
"How to Brew" by John Palmer states in number 2 of "Beginning the Boil."
2. Add the malt extract. Dissolve the 2.5 pounds of dry malt extract into 3 gallons of cold water in the brew pot. You will be boiling the hops in this wort.
In my opinion, dry or liquid, add it in early to start, then a late edition at the end of the brew.
For example, as soon as your steeping grains are done, add in half your extract. Then add in the other half on flame out. I've seen and done it both ways and it works just fine either way. Like what Billy Klubb is saying.