My only concern with running the arduino directly off the panel would be that the panel could output more than 5v in bright sunlight. I have a few panels rated at 1.5 and 3 volts, and in direct sunlight, they will often output up to 2.5 or 4 volts, respectively. Whether or not an extra volt would cause damage or not, I couldn't say, but I personally would prefer to go through the regulator to be safe. The stability issue that I was talking about would be from passing the 5v panel through the regulator (connecting it to Vin), which would drop the voltage down a volt or two.