In most cases, Speed Thunder is pretty much a requirement. It gives near instant Elthunder, which is a good projectile in general. The faster charging allows Robin to better deal with speedsters that tend to dominate the upper tiers (Shiek, ZSS, Diddy, etc.), Although Speed Thunder removes a lot of Thoron's killing, Robin has enough other killy mechanics that it doesn't matter (Levin Sword for one). Thunder+ is really good for team battles where you can just sit back, charge and fire with impunity.
The other customs are either just not worth it or situational at best.