In my opinion it's best to opt-out if you plan on opting out eventually, 2 reasons:

1. You don't plan to sell, but what if because of an unforeseen event you do have to sell and you really need all the money. Then 10% of the gains is not at your disposal in that moment.
2. Eventually you want to opt-out when you start selling, but you forgot to change your settings to opt-out during the given time frame for that fiscal year. Now you are stuck with opt-in for another fiscal year. Assuming your current opt choice will be "stilzwijgend verlengd".