No..
but is best to do it at the end of the boil after heat is removed to lessen the chances of scorching the DME..If you do it while heat is still on the boil would be best to make it a 2 person operation, 1 pouring in the DME and the other stirring the wort to insure it is mixed and doesn't fall to the bottom of the kettle to be scorched.,..I have done it both ways with good results..