If you change coax length and it affects your SWR in minute amounts, everything is working fine. No, there isn't a magic certain length that will do this. I am only refering to newer solid state radios with transistors. Old tube radios usaually have devices built into them to tune the radios finals impedance to match that of the input end of the coax. Other more appropriate names for the antenna tuner are the transmatch or feedline flattener.
Many operators take half the truth of transmission line theory and make up their own rules.
Sorry to admit I thought coax length was important. This isn't the easiest part of CB to wade through. Hopefully I've covered this with enough detail to set everyone straight. Many beginner amateur radio operators and students have misceptions and make false extrapolations in tranmission line theory. There are many conditions that must be stated when simplifying things.