If you had your head rebuilt they should have cleaned up the valves a bit so that they seat better. I've have my head rebuilt twice already and both shops that I took it to did a 3 angle valve job to get the valves to sit a little better. So it may already be done. If you want to, since your head is off, is toss in some flat bottomed valves. Good for compression, probably lighter than stock (depending on the material), and it doesn't toss of cam timing like milling the head does (which you might wanna do a little too).