No, use it for sure. You can just plug in the values for your water individually where I just set it to 100% for the RO dropdown. Then depending on your grist and each lovibond rating, and your water volumes (be really accurate on these values), you'll see the pH that your water alone will bring to the beer. For some styles you'll likely need to adjust your pH up or down to put you in the right pH range. If your water is good though (I'm from the Midwest, where alot of it isn't), then for some styles you may well not need to adjust at all.
EDIT - Obviously, once you brew a beer and go back to brew it again, if there are no changes to grist or volumes, you'd be fine to use what you used the first time if you liked the results. That's where taking good notes comes in. Good luck !