It’s good advice not to request likes if possible, and especially to avoid asking people to “like if you agree” with something. The former makes you seem desperate, and the latter can feel like extortion. If your content is good, people will like it organically.
And now this advice is even better: Facebook is trying to tamp down on these kinds of posts, and has once again tweaked its algorithm to reduce their exposure to followers. Read here for more info.