I think tipping is personal understanding.
It is not mandatory, neither should it be forbidden.
If one feels the need to appreciate someone for his/her work done,then the person should go ahead and do it without finding reasons.
Though, I totally understand what your professor was trying to explain.
Tipping is not necessary and it's a person who decides when and where should they tip to someone who they think that they deserves to have a tip.
I do tipping whenever I feel like it's worth tipping for and that's what I feel about tipping. Some may like tipping some doesn't like it but the best interest will be the one who receive the tipping because they will be much more happier than no tipping at all and a gesture of a small gratitude about their hardwork.