when should buyer sign assignment contract

[Mario T.]

Just curious for people that has done tons of wholesale deals.  

When do you get the end buyer to sign the assignment contract?  One way I've seen it done is to get the end buyer to sign the same purchase contract i signed with the seller except change the purchase amount to include my fee and change the seller and buyer names to my name and the end buyer name.  Then a day before closing get the end buyer to sign the assignment contract.  

Is that something typical wholesalers do?