It depends on the app you use how much data from past cycles they will use to predict the current ones. I imagine some do give you predicted fertile days based on previous months, which would be meaningless if you were on the pill. If you don't know how yours works, yes, I'd just delete it. But also I found that CM was a good predictor of fertile days for me, so that was something I could follow day to day to give me an idea, and my ovulation days were the same both months (I came off the pill, started charting right away, and was pregnant the 2nd month). So once I had a good idea from doing the first month when I'd likely ovulate the second month, I planned for that date. CM seemed to show that I was coming up to being fertile then too. And then I got pregnant that month. I'm not sure what data it really used from the month before, but basically I just estimated based on the cycle day I ovulated the previous month and that was enough. Charting is kind of a pain though, so if you aren't TTC now, I probably wouldn't waste time on it, but that's just me.