My Samsung has the pincushion effect, and the serviceman told me it's a simple replacement of the optical block. Is this true, or is the tech just screwing me around?

Jussssst curious ... you wouldn't happen to be from Albany?

Anyhoo, people have had servicefolks tell them all sorts of things about why pincushion exists, how to get rid of it, why you can't get rid of it, etc.
If the degree of variance from the center of the screen to the top is more than 1/4", then it's considered unacceptable and Samsung will try to fix the problem ... but again, I'm not quite sure how they do it. Some folks have had good success pushing on parts of the bezel (which is somewhat flexible) to reduce the amount of visible pincushion.

There is no optical block in a Samsung, optical blocks are Sonys problem. Samsungs have light engines. But the pincushion is common and as explained by rnick, usually not fixable unless very pronounced. And the "new" light engines are refurbished. So if the pic looks good and the pincushion isn't too bad, keep it.

And if the tech thinks Samsungs have optical blocks, try calling someone else.