I'm pretty sure most speakers were designed with tweeter height as a priority. Most speakers put the tweeter above the mid/low driver(s), some put them under, some put them off to the side. It's not a random placement of what looks best.
Most recommend the tweeters at ear height. Some people are sensitive to that, and prefer the tweeter above or below that. Room acoustics have a big impact on this as well, as does toe in.
At the end of the day, no two room, speakers, and sets of ears are alike. Experiment and use whatever works best for you. Trying them on the floor and tilting them up doesn't cost anything if you use stuff around the house to get an idea of what's going on. Same can be said for raising or lowering the speakers.