For any given gene, what ultimately determines which dna strand serves as the template strand?

The answer is the base sequence of gene's promoter.

DNA is a two-stranded helix. One strand is called a template strand, the other one is called a coding strand. The template strand is copied into mRNA, and the coding strand is complementary to the template strand. RNA polymerase, an enzyme responsible for transcription, has to recognize the right strand before transcription begins. So, it recognizes one or more short sequences on the coding strand. This is the base sequence of gene's promoter. Thus, the complementary strand is the template strand.