One of the most competent aikido Sensai I ever trained with turned up late, didnt warm up (said- its been years since I warmed up) and started the class by saying "dont ask me where I was or what I smoked last night".
I thought I was hearing things but two other people later said they heard the same thing.
It certainly didnt seem to effect HIS ability, understanding, focus, etc. His skill is acclaimed, and not just by aikido people.
But thats HIM, not you. Like everything, it depends on the individual. If I smoked marijuana before training I doubt I would end up turning up there ... I'd probably stay at home and look at the view.
What I dont like is getting that odd wiff of beery breath off certain individuals at times.