When are you supposed to eta-reduce?
$\eta$ conversion is not a mean to reduce a term to $\beta$-normal form, but a tool to show equivalence regarding the (future) application; to express that 2 terms “are the same function”; that they will reduce to the same term when applied to arbitrary term(s).

If you want to also do $\eta$ conversions, you can do it anytime because you still have the same term (application-wise) after performing it. Having $\eta$ conversion in the theory will not enrich nor deplete the theory regarding existence of $\beta$($\eta$)-normal forms.