It depends on the game. Back in the day, I usually only played the routes of the guys I liked. But there was one game where, even though I liked the whole cast, I still only played one route because I wanted the MC to belong to him alone. That was The Second Reproduction, I just loved Gardis way too much ;;;A;;;
Now, since I want to write reviews, I try to play more routes. I generally want to complete the full game, as long as the guys aren't too far off from my taste. For games that force you to play every route to unlock a hidden one, I'll just skip the guys I'm not into.