Heatsink question

[Pagglit]

I have a Thermalright Sk6 heatsink and i want to know before i install it if its bottom surface is supposed to be flat or not. Because it is not perfectly flat nor is it slightly concave(which is suppose to be a mark of a good heatsink).

 

[SWScorch]

flat = good
concave/convex = bad

If its concave AT ALL its not getting good contact with the core. Ideally the base of a heatsink should be as physically possible. This is opne of ther easons why people "lap" or sand their heatsink bases; the other is to remove the tiny grooves/scratches caused by the machining.