It COMPLETELY depends on the quality of the block or heatsink.
I've bought quite a few "quality" heatsinks where the bottom was NOT flat, I took it to work, had the machine shop grind it down flat.
I've seen some blocks that weren't flat either.
Another part where lapping comes into play is when the bottom surface has rough machining marks, this also needs lapped.
Some companies take the time to produce high quality products, with attention to these details, others don't.
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