When I first started playing Pokemon thirteen years ago, I almost never nicknamed them. Mostly because I thought it was stupid that calling my Charmander 'Lizard O Deth' *I was five xD* was silly, because it just said Charmander anyways when it talked *or so I pretended*.
But nowadays, I usually tend to nickname it, mostly because of the Nuzlocke challenge having inspired me to become more attached to the little sprite characters. I'll name an Oshawott after a sea monster, like Nessie, or the male version; Champlain. Or Poseidon if I see fit.
Or maybe a Raichu gets something a bit more specific/punny, like Sparks, Bolt, Jolt, those kinds of things. It's really all about the Pokemon, and what they remind me of, which is how I name them.