While I will agree that setting yourself up to get to all the notes easily is always best, and timbre should always be taken into consideration, I get what the teacher was saying. In the end, all that matters is that you play the right note at the right time. You don't always get enough time to assess the proper position when you're sightreading charts on a gig, and I've made pretzels out of my fingers on occasion. But all anyone cared about was that I hit the note, and I always managed to do it...well, not always...once in a while I goofed it good BL's in that situation really don't care how you kill the cow...they just want the steak. So prep the best you can for it, but just know that occasionally you'll have to make pretzels. The only reason the bands that I've worked with ever require a 5 is for doing "Uptown Funk" by Bruno Mars. I've got a Hipshot double stop XTender on my E string that I can tune to drop D or B, and that always satisfies anyone's occasional need for those low notes. But I can't do "Uptown Funk" on anything but a 5 and have it come out sounding good. Other than that I've done a grat job avoiding it.