Q. How important is when calling someone to the Torah to start with a good subject as we are so careful when finishing the aliya? Is it equally important?

A. Remah (O.H. 138:1) writes that one should always try to begin with a favorable and propitious topic. Mishne Halochos (6: 32) quoting Eliahu Rabba, maintains that the reason is to avoid that the attendants should come to a quarrel, and it is only a good advise.
Poskim write that when they are many aliyos to be given, one can be lenient and begin with an unfavorable topic as long as one finishes well. (See Eishel Avrohom 138, Torah Lishmo 376, Piskei Teshuvos ibid 4: n. 20).

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a