Batches thing is what it is. Even if you wait 3-6 months to buy you are not guaranteed to get one from whatever slightly modified process they may or may not do. Also there is no guarantee they fix anything at all.
The iphone 5 and 5S all had the rattling power button down to the last one made probably. Some on launch day had perfect power buttons and some made 18 months later still had the rattle.
Just buy and enjoy