Fare la cresta, definitely is not a good thing: I'm going to explain why:
Imagine this situation: you and your friends decide to have a party to spare some good time together
The shop list is done: food and drinks, everyone share the same amount of money and Angelo, a friend of yours,volunteers to going to buy everything you need.
At the end of the day Angelo spent 90 euros out of 100 you put in the petty cash.
He saved some money and bought everything was on the list.
Good Job Angelo but wait...
Angelo looks the other way and keep the money he saved.
In this case Angelo ha fatto la cresta: he was smart to save the moneys and buy everything on the list but he kept the change only for him.
The modern meaning has nothing to do with the orignal one
Cresta in Italian means crest
A long time ago farmers used to season dishes with a sour spice called agresto
The vineyard's owner gave the permission to the Farmers to pick up unripe grapes.
From the juice of the unripe grapes they had the agresto.
Sometimes someone used to pick up the good grapes as well, without permission.
And this was called fare l'agresto: when you take something more than you should.
Through the years fare the way of saying fare l'agresto has become fare la cresta.
The two words (cresta and agresto) have nothing in common, they are similar just phonetically.