For the majority of geographic areas, especially those expected to be undergoing continual population growth or other demographic changes at a relatively steady rate, it may be better to perform a linear regression on the annual ACS data with respect to year, then interpolate or extrapolate to the year of interest, than to average the annual ACS data. Obviously there are other areas that have experienced significant discontinuities in population or demographic characteristics for which this method would not be very reliable, but then neither would using multi-year ACS averages.