Yes the exact longitudes of planets are taken with respect to the Ascendant for calculating the strengths of planets and bhavas. Divisional charts were necessary when there were no computers and manually calculating the strength of planets and bhavas used to take 80 man hours per horoscope by a Mathematician which was not a practical proposition. So some amount of approximation was required to apply the axioms in astrology and divisional charts were of use to us then in the olden days. Today with the advent of computers and relevant software, we can do it in less than 15 seconds.
I have been practicing Astrology for the last 50 years and I am familiar with the old methods as well as the new computerised calculations since basically I am a software architect too for as many number of years.