it might depend on the situation. If they give you a bunch of numbers and ask for the standard deviation (sd) then absolutely use a calculator, there no way they'd expect you do it by hand. if they give the variance or the z-score where the formula is on the formula sheet, then use the formula. they aren't hard formulas so you shuld be fine.
most of the time finding the sd is only 1 mark, so they'd have to give to you if you're right
(i'm assuming this adv, but i think standard should be the same)