If you’re a programmer, you’re writing programs for your users. You are there performing a service for your users, not the other way around. Here’s an excellent way to insult your users by letting them know that they are not worth one minute of your time:
The parenthetical (s) as a shorthand for saying “This could be singular or plural and still let me off the hook” is a throwback to times before computers, when people worked on paper forms. It is now unacceptable.
Is it a pain to write that out? Hardly. Maybe a twinge of discomfort at worst. However, to take the shortcut of (s) says that you are so lazy as a programmer that you don’t care about the user, and it says that every single time the user sees that message.