When you do addition or subtraction, you follow the least d.p for deciding how the final answer should be written. For instance, a = 5.0 (1 d.p) + 7 (zero d.p) = 12 (zero d.p), and not 12.0 (wrong answer).
When you do multiplication or division, you follow the least s.f for deciding how the final answer should be written. For instance, V = 5.0 m then you will get 1/V = 0.20 m-1.
Computation of angle: 2 d.p in working steps and 1 d.p as your final answer. This is different from computing of numbers where you presented the 5 s.f value before rounding off to required s.f for your final answer. HOWEVER, do note that if you are measuring an angle in your practical, using a protractor, then your angles should be given to whole numbers during recording.
Computation of cost/money: 2 d.p in dollars or whole integer when done in cents.
When taking average, you follow the d.p of the data. For instance, finding the average time taken of t1= 24.5 s and t2= 24.7s. Then t_ave = (24.5 + 24.7)/2 = 24.6 (1 d.p, not 3 s.f)
Find period of a pendulum from average time taken of 20 oscillations. T = t_ave/20. In this case, T follows the s.f of t_ave. Not d.p.
The value of trigonometric function: sin (45 degrees) = 0.707; sin (45.0 degrees) = 0.7071.