You must always make sure that turning all the LEDs on 100% does not cause damage to anything
Without good bypassing, you would be lucky to even have the chip operate past reset!
...but they often do work in simple cases...
Like when bread-boarding. The metal plates in the breadboard provide enough capacitance to take the place of actual capacitors.
Why a stock Arduino is not set to full swing is beyond me.
On a (sort of) related note, I've found that Arduino boards like the Uno and 2560 are quite sensitive to the oscillator stopping if the board is held in a bare hand.