Hardening off

I just read a bit in a magazine from a well known gardener where he said you didn't need to harden off plants before planting out, you just planted them then to protect them, cover with fleece, I have never tried planting straight into the soil, but have moved them out of the greenhouse and protected them at night with fleece, has anyone  just planted then protected straight from the greenhouse and if so we're you pleased with the results?
Just to add, would it work with potatoes