That depends on how you engineered your shield, what shield you're using, what ship you're using and how many shield boosters you're fitting. It can get a bit complicated.
This.
Best thing to do is have a play with Coriolis to see how different shield/booster combinations stack up, paying particular attention to resistances as well as shield capacity.
If you go for a Reinforced shield mod', for example, it'll have a high(er) capacity but low thermal resistance - which means you might want to fit a couple of boosters with a thermal resistance mod'.
Conversely, if you go for a Thermal Resist shield mod, it'll have a lower capacity but super-high thermal resistance - which means you might decide to fit Heavy Duty SBs without having to worry about having low resistances.
*EDIT*
FWIW, personally I usually go for thermal-resist shields with the high-cap XFX and then I go for an even mix of HD SBs and RA SBs, adjusting the mix slightly depending if I think I need higher resistances or more shield power.
Having done that, I either bail out of combat when my shield gets low or, in an emergency, I use reboot/repair to get a shield back on line in a hurry.