I'm late, but I hope I can help. The line above the vowel is called a macron, and it's the most common way of marking a long vowel in Latin nowadays. There have been various ways of marking long vowels throughout history, but it's also been very common to forgo them. Basically, it's up to the writer to decide if they want to make it explicit which vowels are long, or if they leave it to the reader to know which vowels are long.
So you will typically find macrons in textbooks and other teaching materials, if the author wants to make sure the reader learns about vowel length. On the other hand, a lot of edited original Latin texts will not include them, since those editions are often aimed at an audience of scholars who should be able to tell which vowels are long. They might explicitly mark a vowel as long or short if it's an unusual case, though.
So if you're worried about doing it wrong, please don't. As long as your usage of macrons is consistent, there is no wrong choice. It's a matter of taste and preference. Though if you're still learning, it's probably a wise choice to keep track of vowel length.
I'm puzzled about one thing, though. You say LLPSI doesn't use macrons, but it's actually pretty well known for using them. Are you confusing it with another textbook, by any chance?