I add several layers of tape under my grips but only where my left hand grips the club (I’m a lefty). I know there’s not supposed to be a bulge or waist on your grip, but does this slight difference make my grips illegal?
Let your conscience guide you. The rule states no bulge or waist. It does not identify size or whether it is obvious to the naked eye. If your goal is to build a bulge into the grip that might assist you in making a stroke, then you are violating the intent of the rule.
I had a case a number of years ago when I was technical director of the USGA. A grip designer asked if his grip conformed. He wanted me to look at it to see if it had a bulge. I asked him if, as the designer, he had designed a bulge into the grip. He admitted he had, but it was so small I might not be able to detect it. No more communication was necessary.
To help you further, if you are interested in reading more about how the rules regarding the grip (and other implements) have been developed, for some fun summer reading you should order a copy of my book From Sticks and Stones .