Trading Around a Core Position

I do short term stock trading. I typically swing trade. But, sometimes I'll do a day trade also. Regarding trading around a core position. Say for example, you initiate a core position at $75/share. Then it goes to $65, and you add more. When it gets to $85, is it more common and/or better to sell the $75 shares or the $65 shares? If I had to guess, I would say that selling the $75 shares would be better because then you have lowered the cost basis on your core position to $65. I'm hoping someone who does this kind of trading can let me know which method is more commonly done by professional traders/fund managers. Thanks.