On crops that take a super long time like Thav Onions, I totally think it's worth fertilizing. You can easily fertilize them 24 times over the course of 8 days (that would be 3 times a day), to cut the growth time down by a day (to 9 days). If you can fertilize them more often, then you can cut them down even more. On most crops, I'm in the not too worried about it camp.