im sorry, a bad batch of fuel happens once in a blue moon.. not every time you fill up.. no direspect to your tuner, but if hes saying its bad fuel every time, then maybe its worth taking it elsewhere for a touchup tune. retarding the timing a few degrees can help, but your removing power and changing the tune.. i wouldnt be doing this without having it on a dyno to see the impact its having.
take it somewhere else to get a touch up. if they cant make it work, at least you know your tuner isn't full of hot air. then you can start considering other possible reasons.