If adult games had achievements/trophies, I'd be 100%'ing all of them. But since I rarely ever get invested in a single character enough to sacrifice the rest, I haven't strayed from the harem paths to date.
I'm getting all my girls. The only reason I probably wouldn't would be because there's some serious downside or I really disliked one of the girls.
That or Forcing a dude into the Harem end, my bros cannot be my hoes.
Harem all the way, but I am a completionist.
I will scan through comments or walkthroughs to see if the non-harem paths have any content variations not in the harem path, e.g. being monogamous gets you extra activity with your chosen partner, or leaving out one partner has another fill their place in group scenes. Then with a bit of save scumming, I'll try and get each variant.
What seems to be the case is that leaving out one or more partners just cuts down on the scenes you see, so the simplest way to see all scenes is to do the harem route.
I always go with the gotta catch 'em all route.
If it has something that might interesting content if I don't go that route I might try something different in a second playthrough.
Otherwise I will always choose the harem ending.