Is it OK if I do not take the receipt in Germany?

It's absolutely in Ordnung. If you don't need it, you don't take it, as many people in Germany do.

In some countries it is apparently easier to evade paying taxes if you don't print a receipt for your income. In China for example, businesses for a time were required to get a special printer and print receipts on government issued numbered forms using a government issued software application and for a while these forms even had a scratch area that would reveal hidden prizes, in order to motivate customers to ask for that receipt. Nowadays it's more and more electronic though, and printed receipts are no longer required everywhere.

In Germany that is not the case. Taxes are only based on the businesses own records, and a copy of the receipt given to the customer is such a record; however, whether you keep your copy is irrelevant. The records have to be complete. Every transaction has to be recorded, and skipping transactions can likely be found in a tax-audit.

Though I don't know if it is actually harder to cheat in Germany, or if it is simply done so rarely as to not be an issue.

I live in Munich and receipts here are generally printed on-demand, i.e. the cashier asks if you need the receipt and if the answer is yes, he will print it for you. But:

• Now-defunct grocery chain Tengelmann used to put up signs suggesting that customers don't take a receipt if not needed to save the environment.
• Waiting for the receipt to be printed may increase waiting time for the people behind you in the queue.

Hence it is perfectly fine not to take the receipt.

In Germany, this is perfectly alright, especially in a "fancy store".

The receipt is obviously needed should you ever want to return a good (hardly the case with chocolate), though in a "fancy store", your word of honour is normally enough anyway (also, shop assistants will immediately recognize your face and know what you bought -- even after weeks, not sure how they do that, it's scary). Not so in "mass" shops. No receipt, no return, no warranty.

You may want to keep the receipt (and in particular, not throw it away in front of the store!) just in case because there exist singular cases where people have gotten in trouble due to scammers. Some 10 or so years ago, it reportedly happened rather often that someone would pick up your receipt at a supermarket, and then stop you while you were loading stuff into your car, call police and claim you stole their goods (providing the receipt as evidence).
I'm doubtful about whether that would actually hold in court, especially since it's pretty obvious from surveillance tapes and from your credit card bill that you paid for the goods. But reportedly, scammers were often successful with that (maybe also because people didn't want to risk a criminal charge?). Well, whatever, that's that. It's easy to avoid that possibility by just taking the receipt with you.

You may want to keep the receipt because some (not all) shops print more or less obscure transaction data, and sometimes data that can be used to identify you or your card on the receipt. If your paranoia level isn't above threshold for that, it's no issue.

You may also want to keep the receipt as a proof of ownerships in some stores where the cash point is very far from the exit. Otherwise, an overzealous in-house detective may become a nuisance if you can't present the receipt (though it has never happened to me, personally).

Nowadays, most stores do not print the receipt at all if you say "No" after they ask you REWE being an obnoxious exception: They ask, and you say "No", they still print it, and put it under the table quickly afterwards, but then make a vexed face when you say: "OK, now that you printed it anyway, I'll take it with me.".

In other places, by not taking the receipt you risk severe trouble, notably in the German-speaking northern part of Italy (Alto Adige). Eat in a restaurant, pay your bill, fail to take the receipt, and walk away. A hundred meters down the road, police stops you, and you're in real trouble as accomplice in a tax fraud.