Discipline where necessary, slack whenever needed. The smart thing is to know where/when to apply what and to what extent.
Because discipline is not something that can be relied upon, I much prefer the idea of creating systems that strengthen virtuous habits that over time cement useful behaviors that “automatically” lead to the desired state.
I am especially wary of discipline as a “solution” because I have seen first hand the mess and learned helplessness a manager calling for discipline can create when he doesn’t in parallel lend a helping hand for the team to gradually establish the “scaffold”/trellis on which something good can grow, with or without relying on fluctuating discipline.
I have also seen the massive positive impact you get when the team (including the manager) uses the scarce resource of discipline to build the “systems” I mentioned above, so that continuous improvement does not need to be perennially fueled by a scarce resource.
In many ways this is related to Lean and to what George Kohlrieser calls “secure base leadership”, but it’s cringe-worthy to call oneself a leader, so let’s leave it at that.