It can help a little with fuel economy. This 1936 research paper shows an increase in thermal efficiency once a 300 degree F threshold was reached.
http://aerade.cranfield.ac.uk/ara/1936/naca-tn-565.pdf
Note initially results showed a decline in efficiency until the 300 degree F thresh hold was reached. This may have been due to the low compression ratio engine used (13.5 to 1). With a higher compression ratio engine the threshold would be at a lower temp. Also note that the fuels compressibility changes with temperature and this can have an effect on injection timing.