If 3NT by agreement does not guarantee a stopper and you know that, you certainly should alert. If your bidding agreements make a lack of stopper more likely than normal, the opponents have a right to the result of your agreements. If 3NT might not have a stopper due to the situation rather than your agreements, then you do not alert. I suggest that you explain your reason for not alerting instead of a immature lol. In my experience, when someone resorts to rude sarcasm he is unsure of himself for some reason.