i lapped mine, had bad machining marks which im sure they would interfere with heat transfer. though at home depot they didnt have any higher than 600 grit so i didnt get to make a mirror finish but it was a lot better smoothness wise than before
unless it has machining marks on it (lines across it) i wouldnt bother lapping it, or if youre afraid youll mess it up which can very well happen just leave it be.
if you do lap it, get some 250(if big marks) 400, 600, 1200 and 2000 if you really want and just spend like 15-20 minutes on each one except maybe the 250. do it in figure 8's turning it a bit every so often and do it on a flat surface (mirror, glass, table) i used a piece of wood i had layin around thats like 1x1 feet and was very flat and smoothe.