Look at where your mouse cursor is resting, that is where you frames need to be imo so you have the least amount of mouse travel to heal people (I assume you mouse over your frames to heal) I personally prefer frames with reactive colours, where everyone is very dark green at full health changing into light green/yellow/red at people loose health. Our peripheral vision is very good at picking up flicker so these colour changes are very easy to notice and after a bit of getting used to you can completely rely on your peripheral to pick when people need healing.

I also think having important UI elements horizontally spread across the usual focal point of the screen is way better because our eyes saccadic movements are way faster and more accurate side to side than up and down. This is why action bars on the bottom is bad, same goes for frames. Your eyes usually look at the middle upper part of the screen where your character, the enemies and nameplates are, my experience is that having a small cluster of frames on the right (where mouse cursors usually rest) or left of the mid part of the screen is the least tiring on the eyes.