$1936

I am not very experienced in making bracelets and I would like to know how to make a perfectly straight bracelet, without curves.

You need to practice a lot. Even tension is the most important thing you need to learn.

kleinevos, would you say you generally tie your knots quite tight? Lately I've been practicing with loose knots that make a softer and floppier bracelet, but also much less wavy on the edges. Is that a good strategy?

Too tight is not good, too loose is also not good. As long as your tension is even, the bracelet will be nice and straight. It’s always good to experiment with your tension till you find what works for you. So yes, your strategy is good.
I tie pretty tight, but that doesn’t mean that works best for you, too. I know some people that tie much more sloppy bracelets that are still very nice and straight.