I agree, I have missed more than 4 temps along the way and FF still detected ovulation. I think if you have three temps in a row that are a certain amount higher than the average before that it will call it ovulation. I'm sure it's more complicated than that but that's basically what I've noticed. I have also charted only temps some months (no cp/cm) and it still detected the O. If the temp shift is clear it will give you at least the dashed line.
So I say go for it and it's still worth charting! Good luck!