The GCC compiler is pretty smart and it should convert the % to & automatically during its optimization phase. Try it: I bet the code generated will be the same size with both approaches (as long as the modulus is a power of 2).
Knowing this, it's always better to write clear(er) code than to prematurely optimize it.
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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons