How to treat unused pins?

I have read different opinions on unused pins and would like to know what the thought is here on this forum?

I am using a Arduino Nano and reading a MXP5100 pressure sensor. I get some fluctation for my zero and wonder if it is from noice.

I have read unused pins set for inputs, output, high and low, resitors and internal. Does anyone know what is suggested by Atmel?

Thanks,
Kevin