it's preference, but I'd say it's time to ream if it's thicker than a dime all around. I also don't try to"help" my pipe build cake, it builds itself in time. After each bowl, regardless of whether I'm breaking in or not, I put a palm over the bowl and shake up the ash to evenly coat the bowl and then wipe it out with a pipe cleaner once it's cool. This gives you an even cake build up and a dense cake to boot. Not cleaning out your pipe every time and trying to build a cake fast will give you a softer cake that is lumpy and uneven....in my experience.

I have used this method forever and don't ever need excessive or frequent reaming and all my pipes smoke nice and dry, even the ones that used to smoke a bit wet now smoke dry with a good cake. Let it happen naturally.