In a perfect world, all windows of a suburban house would have a bird preventative. However, if that is not feasible, target the most problematic windows to help the most birds:
To figure which windows are the most harmful, record any dead birds you find, outside and near a window, and the thud! of a collison. You should also step outside at different times of day to note which windows reflect the sky, the ground, or any surrounding vegetation. Also, be on the lookout for the “Dark Hole Effect.” This is when a window appears black and dark—this also attracts birds.
Then look at your observations. Did you find more dead birds under one specific window than others? Do you always hear a bird hitting a window on the north side of your house? What are your windows reflecting? Determine which windows are the most harmful based upon this data.
Even if none of your windows seem worse than others, still pick some windows to apply preventatives on! (A judgement call on which ones). You can always reach out to your local birding organization for advice if you want more guidance.