People decide to exercise for every reason except the one that matters most at filing time. The company is doing well, a window has opened, a colleague exercised, the options are about to lapse. All real. None of them is the question we would ask first.
The question is: which year will this land in, and where will you be resident that year? Exercise fixes a value, a withholding and a cost record on a particular day, and the year that day falls in, together with your residential status for it, shapes almost everything that follows. Someone who exercises in March and someone who waits three weeks into April have made the same decision in two different years, and sometimes two different countries. The gap between them is not the market. It is the calendar.
So before the mechanics (the value, the withholding, the reporting, all of which the matter page sets out), settle the two facts that sit underneath them. If you are weighing an exercise, tell us when you are minded to do it and where you expect to be living. That is usually enough for us to tell you whether the timing is working for you or against you.