Where I live (and other countries I've lived) it's commonly stated in any contract that landlords can not show up without a 48 hours notice.
As it should be: the moment I'm paying rent that's MY house.
Also, I'm 99% sure that if you enter the house unannounced while people are at work (landlord or not) that's breaking and entering, so all in all I think yours is really bad advice and you should never do that.
Paying tenants have the right to consider the house as their property as specified in the contract, and have the right to act towards home invaders as they see fit.
In particular, if I got home and found you in poking around my shit I'd let my dogs have a go at you.
The only common sense bit is that yes, you shouldn't rent to unemployed, that crazy.
Also in my country people usually have to provide proof of income when renting, so I don't know who would do that.
Edit: fuck, that's unacceptable. Surprise visit? I might be doning anything: showering, work, sex, having dinner, or people over. I would definitely slam the door in your face.