Yeah castrating after killing would do precisely nothing for meat quality. THey need to be raised "fixed" or else they'll be more tough and I'll be nice and call it "gamey". If you read about a boar's mating habits, you'll understand why. I won't go into detail here. Suffice to say, they undergo a lot of strain and high tension, and this carries into the meat characteristics.
Females (like in deer also) give you the best chance of good meat, then a castrated male. Now this is different than market hogs, and a wild boar lives a much different life than a market hog. So I'm sure there are exceptions to the rule.