Never! Not unless you're fond of disintegration via giant laser beam! Execute triangles,ngons and complex poles with extreme prejudice! Jokes aside it comes down to prefrence and practicality. Leaving a triangle, ngon or complex pole in a mesh that doesn't require subdivision ( in that case, evenly sized and distributed quads are king) is not going to make or break anything especially on a flat surface. Now as personal preference and modelling etiquette I model Purley with quads, whether modelling organic or hardsurface. Mainly this is due to using subdivision surface modelling techniques for all disciplines, as opposed to manually bevelling etc. They'll be times where you're on a short deadline and just need to get the model done and sent off, or it's something the client is never going to see, and you're just delivering a final render etc. These occasions an argument could be made that Purley quad geometry isn't required if the end result comes out the same. For me personally topology is a fascinating puzzle so I'll always model in quads. Something about a clean flowing quad mesh that really catches the eye. Are pure quads always 100 percent required? No. Is it good practice to do so? CERTAINLY. V