I think that w/3 builders then you will be good to wait till your Def turrets are finished. esp since you can keep them all busy.
I would however, recommend that you build them while your TH is upgrading after your defenses are built to max ( or stagger the builders out while you upgrade walls). They help a lot at TH8-9 and though some have said they like the short build time and can balance out builds in later levels.. the sheer HP value they bring is enough to get them done while you transition to TH8 IMO..
They provide a nice chunk of resources that you do not need to break a shield for, drop a troop to obtain, and will not get revenged by someone for collecting them !

