Tank looks nice. Great piece of wood. Plants look healthy.
I think what Sean is talking about is a landscaping/art principle called - what? - the "golden ratio" or somesuch? In short, our eyes find things more pleasing that are not entirely centered and symmetrical. You can find more info than you ever wanted about it over at Aquabotanic.
Why do you use RO? I don't know anything about rainbows, but Lake Mich water is pretty good for most aquarium purposes.
Anyone else wanna bet he loses the rocks as he adds more plants?
You asked about whether you need CO2 if you stick with microswords, etc. All I can say is that I am so happy I started CO2. No comparison between before and after in terms of plant health, groth, fullness, etc. But a 75 is probably too big for DIY CO2. And you might want to do something about the surface agitation from the Emperor.
Tho you mention sticking with "grasslike" plants, I think you definitely could use some variation in your plants. Some of the things I could give you that I think would complement your setup include:
(To add different colors and different leaf-types)
crypt wendtii bronze
red sunset hygro
(to add leaf type/texture variety):