Should I only add people on Facebook if I know them personally in real life ?

Per TOS Terms of Service Yes If you add Strangers and they are Honest and dont accept it and Mark it as Spam or that they dont know you Facebook may Block you from Sending Friend Requests Temporary 

Usually first time its 48 hours 

2nd time you get Blocked 7 days 

3rd time and After 30 days 

You don’t have to! But the person will most likely get creeped out and decline. 

That's a perfectly reasonable stance. Outside of some odd relatives I don't have any FB friends I've never met. I'd make an exception for people I've interacted with enough to respect.