I always say race up. But, it doesn't matter. Do what you enjoy and have fun with it. It's not like you're riding off the front of a B race every time you line up. That's the type of thing that should result in automatic upgrades. Racing smart and having everything align for the win makes it sound like the Bs are a really good category for you at this point. If it ever becomes "easy", then start looking towards full devotion to the As.