My Rule of Thumb (ROT) :
- intended Combat or Defense against a single Engagement? A-Grade Shields or Prismatics if you got 'em. Shields designed for maximum total strength and egress is to be made shortly before but no later at some time after losing them (there's of course exceptions, i.e. very fast Ships capable to perform a valid Separation in order to complete a Reboot/Repair Sequence to re-attain 50% Shields).
- intended Combat or Defense against repeated or even continuous Engagements (long Station time desired, i.e. farming Bounties at a RES)? Take Bi-Weave and ensure reasonable rebuild/regen times via Engineering and Shield Boosters.
In most normal Scenarios, that means Ships not intended for Combat carry potent A-Grade Shields - while Combat Ships carry Bi-Weaves with optimized Regen/Rebuild rates + Shield Boosters to reflect that.
So at least to me, it's a clear choice between a "1-Trick pony" (survive or fight a single Engagement, purely Defensive or designed against High-Value Targets) or the "All-Rounder Combat Design" capable to maintain Station even in very hostile environments. Latter typically requires provisions for post-Shields Combat, thus requiring a severely hardened hull and considerable Module protection - Internals permitting also an AFMU to extend Station time and keep the Vessel in a combat-worthy condition.
Engineering and specific Ship Builds of course create a large "grey area", so nothing is set in stone.
There's some crazy and interesting build Options that can safely violate above ROT easily.