when graphing something llike

r = 1- 2cos(t) ( a limacon)

in addition to multiples of pi/2 also locate the zeroes as these are where

r can change from pos to neg

1-2cos(t) = 0 t = pi/3, 5pi/3 so include these values as well as the usual suspects pi/2,pi,3pi/2, and 2pi

when graphing r = a cos(nt) or asin(nt) consider multiples of pi/(2n)

This will give you the values where r= -a,0 and a

For a more complete discussion see Polar Coordinate System