There aren't really pros or cons to using an o-ring on your machine. O-rings will make your machine run a little faster because of the little bit of extra tension that they add to the front spring. But with that, you have to use more voltage to push it ( I think a little extra voltage isn't a big deal) Or you can just use a stronger front spring instead of an o-ring. Sometimes, I've just been happier throwing an o-ring on a machine.
If your springs are running just right without one, rock on! If your machine runs great with one, rock on!
Most hardware stores (Ace, Home Depot) have a whole section full of drawers of screws,washers, and whatnots that tattoo suppliers resale for wayyy too much. In that section is a section full of any type of o-ring you'll ever want for a fraction of what you'll pay anywhere else. They make any size and thickness that you could imagine for 10cents a pop (sometimes less)
Experiment with it! It's all fun.