I used to power the Arduino on the 5 volt pin. I strongly advice against doing so now.
First of All I did have problems. I guess they were engine related which drove the voltage up but I never had a decent explanation.
More importantly though
If you supply power on the 5 volt and you plug in the USB or power the jack you (can?) get into problems.
That is why I advice to power with a modified USB cable. This way you have to remove the USB power plug to plug your USB cable in.
... or better use a switching regulator and power with a modified USB cable. The last option will make your solution run 2 times longer.
It costs an USB cable but it can save your arduino and your shields. So I think it is worth it.