Bearings, like other things, come down to preferences. I suggest people buy some bearing and swap and see what you like best for a particular yoyo.
Some people think bearings make no difference, some people do. I think it can make a difference but it has to be done in respect to preferences.
Like, my Speeder 2 plays best for me with a KK i it. My Meteor ALSO plays better for me with a KK in it. Both come with YYJ Speed bearings, and the Speeder 2also comes with the narrow bearing. My Chaser, I did NOT like with anything other than a flat bearing in it. My Phenom and Phenomizm, I find they work best with the KK that comes pre-installed in it.
For me, the Crucial Half & Half performs best with a Crucial Grooved bearing in it.
My CLYW’s and One Drops, I just happen to line them best with flat bearings in them.
My DM2’s both have Terrapin X Wing Cut Dry Play bearings in them. I’ve tried other bearings in them and this is what I like best in them. I disliked the KK and Center Tracs in them, and the Crucial Grooved didn’t really work for me in them very well. OD 10 Balls were really nice in there for me. But after all was tried, I really liked the Terrapin X Wing Cut bearings in them.
I have some yoyos with CenterTracs pre-installed in them, namely my Protostar and Northstar as examples. I’m pretty pleased with how those yoyos perform and I haven’t felt the need to mess with it.
I’ve recently gotten some new stuff and I’m going to be experimenting with bearings until I find what I find I like best. I like to have at least 1 extra of every kind of bearing so I have the option to experiment. Once something finds a permanent home in a yoyo, I buy a replacement. Sometimes I buy other stuff too and stock up just in case. I will say most of the time I’m happy with the stock bearing. Many times, I’m finding I swap a bad flat bearing for a better flat bearing.
Using bearings to solve a bad throw or straighten a bad throw is not going to address the problem. Only a better throw will help resolve the problem. Once you have this down, you can really start playing with bearings.