I generally don't. I too have driven several manual pickups, and only once had a squeaky throwout bearing. But I bought that one used so who knows what it had gone through before me.
I've never heard of this rule, and I suspect it's more valid for tiny European cars than for pickups. If I want to take off quickly, e.g. third in line at a short left turn, I keep the pedal depressed. If I'm tired, I'll shift out of gear. If neither of those conditions holds, I might shift or I might stay on the clutch.
I generally don't. I too have driven several manual pickups, and only once had a squeaky throwout bearing. But I bought that one used so who knows what it had gone through before me.