With a good 6 week veg under hids, I start flowering and the males do a good job showing smell level, plant structure, vigor and resistance to stress and overall heartiness. Especially if you can get a male of an unusual pheno (it's fan leaves look far more sativa than expected, etc. ). That's just my approach but whatever works for ya! Ideally test crosses of course :)
Since isolating my males, they've been subjected to subpar lighting and sleep cycles, so I've pulled 5 strains that were too weak, but the following three can't be stopped, making them good dads!