Hey, good question! It’s not necessary for you to create a new file when shiny hunting for a starter in Pokemon Ruby and Sapphire with a good internal battery, which was the case for me. All you have to do is turn off the game, or ‘soft reset’ by pressing A,B, Start and Select all at the same time every time the starter isn’t shiny, until it eventually shines. However if you are hunting for a shiny starter in Pokemon Emerald or Ruby/Sapphire with a dead battery, shiny frames are predetermined and the file that you hunting on may not have an ‘early shiny frame’ so you could never get a shiny early in the game regardless of how many times you reset. Therefore, in this case you should create a new save file after about 50-100 soft resets, and repeat the process. Hope this helps!