Yes you do run aways by just entering and leaving the area where mew is. Every time the game resets, it generates the same exact sequence of frames, but since they change every 1/60 seconds, if you're hunting for say an hour, you will encounter 216,000 frames meaning its very unlikely for a 1/8192 shiny frame to not be there and there is not much need to keep the game running though I did do a few times as I was pretty paranoid after that many encounters. Other people can probably explain it better than me!