I don't think it is too critical. I used all new lifters, but with used cams when the engine was built the first time... and I didn't do really anything special to break in the cams/lifters together and there's no abnormal or excessive wear on either.
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Originally Posted by CarCraft
If there is one period of time when a flat-tappet hydraulic or mechanical lifter cam is most susceptible to severe wear, it is during the first 20 to 30 minutes of camshaft operation. This is when the lifter and cam lobe create their initial wear pattern.
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I think there's some sort of possibility of a cam's lobe to be worn down so it doesn't open the valve as far as it should. Or the lifter surface to be worn down such that it's not opening like it should, causing lifter tick/tap...