How do YOU want it to look...?
Personally, I would add more plants, and try to vary the placement a bit so it doesn't look like a row across the back of the tank. Maybe go for an arrangement which slopes down from one side to the other.
If you don't want to add more plants, I'd take some of the ones fro the middle of the tank and add them to the back corners to get a more concave effect.
The vallis will probablly start sending runners out soon so's you could use them to fill in any gaps later.
So much is down to what looks good to you though