I did 10 total moonlighting shifts as a resident, all in my last 6 months, in a town 1 hour away, always double coverage. The extra money was nice, but not critical to my financial success. The experience was okay, but not critical. I do remember feeling pretty uncomfortable, at least initially. In retrospect, much of that was the fact that I was in a new hospital with new systems and consultants, not because I was "on my own."
I always had mixed feelings about moonlighting as a resident. I mean, we say that an EM residency is so important and then we proceed to work without completing one.
Whatever you choose to do, remember that residency is a very important time for professional development. Don't blow it. There will be plenty of time to make gobs of money later. A little bit of moonlighting like I did is probably fine. But if you're making $100K extra a year, or moonlighting as a PGY-2, I think you're probably doing residency the wrong way.
This advice assumes you're doing "standard moonlighting" like I was- busy shifts in a real ED. If you're doing something else- 24 hours seeing 2 patients, flying around in a chopper, sleeping etc then that might work out better.