"Don't underexpose your film"
Not a chance, I shoot almost all my 35mm work pushed 2 stops, and it's beautiful. Tri-X at 1600 or 3200 with a Red 25 is gorgeous.
"Don't handhold a Pentax 6x7"
Pft, that's why they exist, and why I don't use a Mamiya.
Many people who want to bestow some impromptu advice to me on the street about my work are usually armchair photographers who have no idea about it.
Best one ever:
"It's useless to handhold a Hasselblad for night photography"
Not when you've got a knack for pushing Tri-X up the whazoo, and using Delta 3200, it's not.