I used to always go full harem, but sometimes it not worth seeing content with a girl you don't like just for the sake of completion. Nowadays I skip a lot of side girls as soon as they get a little bit annoying,
I'm just about to play No More Money through with only an incest ending. I was wondering if the sister wouldn't bite his dick if this was the case. also hayley is much better with a non harem ending.
In some games the harem route is basically doing all the single routes at once, and you're just missing content if you choose to avoid it. Japanese VNs on the other hand often have the harem route as a sort of bonus thing at the end, which isn't as fleshed out as the normal routes. Generally speaking I'm going to go for the harem route, though it depends on the story. Sometimes sleeping around just doesn't feel in line with the tone of the story.
Virtually never, but will without hesitation if only one LI is interesting or if the LIs are written as being only begrudgingly accepting of a harem. This just usually isn't the case so is a non-issue.