Who is saying this? Always evaluate the speaker. Do they have experience, and actually show the technical work they're advising about? Most of the time, no. Triangles are mostly bad for subdivision surface modeling. Yes tris are ok for deformation, but it's totally dependent on specific topology. Which way an edge flows…
A lot of these sorts of (usually entry-level) questions about triangle usage culminate in the following understanding: 1) The notion that one should never use tris/ngons is reductive, and there are times when tris are actually beneficial to deformation. 2) Quads get turned into tris at the end of the process, anyway. I'd…