Unless you are going for straight out long sleepers?
You don’t need a string, centered. You just need to consistently keep the string from dragging against the wall of a Yoyo on the throw down.
A simple Centertrac was specifically designed to facilitate that end.
There are 3 primary functional bearing styles with legit pedigree: flat bearing… KonKave bearing and Centertrac bearing. All 3 allow for lateral movement of the Yoyo string, with minimal drag/effort.
The KK and the CT allow lateral movement, with the added feature of no or minimal friction contact of string to surface of Yoyo.
Obviously, some V shaped yoyos have such LOW walls or No walls; a flat bearing may actually have an advantage of easiest side movement potential of the 3 types.
No one has any valid proof that center Grooved bearings or V shaped bearing(that immediately direct your string to the exact center of the gap) have any actual performance advantages. <> Center-locked bearings; might keep your string away from the walls(if any) of a Yoyo. But if your string is at neutral or tighter tension; you have effectively divided your gap into two parts. And you can cause ‘drag’ of string against string; as the segments fold into,the gap and the Centered string cannot move sideways.
My experience with center-grooved bearings (Trifecta, Crucial) is that they don’t really lock the string in place anyhow. Your string loop is loose enough and the friction in that groove is low enough that as soon as lateral movement is required, they pop out.
I used to think they’d halve the capability of lateral movement and cause more snags and binds, but testing it out proved me wrong.
[edit: brought to my attention that you can INDUCE tension through play. This may indeed be enough to lock the string in place.]
I do not think they offer an “advantage”, though. There’s definitely no proof that the grooves help any more than the centering provided by a KK. My favourite profiled bearings are are still KKs and CTX (10-ball Center Trac). I keep the grooved ones in my yoyo as long as they play smooth and quiet, but as soon as they get cruddy on me I typically swap in a KK or CT.
When you move the yoyo around, the string will rub the wall more than a concaved bearing can help, unless if you can move very very precisely that the string don’t touch the wall at all (which is practically impossible).
Unfortunately many flat bearings people own are the ones that come stock with the yoyo. Obviously when you compare a rather low cost flat bearing that already been used with a new high end concave, the concave performs better…
The Terrapin X delta cut (centering) keeps the string dead center all the time. It is coated in dryplay which means that you don’t have to lube the bearing, although this makes it quite noisy out of box. Mine did not take long to break in and become quieter though. The sleep time is pretty good. I was replacing my ten ball with the Terrapin, and it did increase the sleep time. It also made my yoyo more stable on the string. This or a Crucial groove would be my best advice for you. ( I don’t know much about the Crucial, but I have heard great things about them, and I just ordered two of them yesterday).