The short answer is no, it is not a good idea. Unless you can stratigraphically match all the intervals in a core (by varves or other markers) and can show that sediment thickness (taking into account compaction of the core as it is taken out of the ground) is the same between these markers, then their is likely to be differences in accumulation rates even in relatively closely spaced cores. If you look at a paper on Lake Mead cores (Rosen and Van Metre (2010), two cores taken less than a kilometer apart or so show a peak in DDE at different depths. When both cores were dated, the beaks were synchronous in time.

Full Reference:

Rosen M.R. and Van Metre, P.C. 2010, Assessment of multiple sources of anthropogenic and natural chemical inputs to a morphologically complex basin, Lake Mead, USA. Palaeogeography, Palaeoclimatology, Palaeoecology, 294, 30-43.

In this same paper you will see that mass accumulation rates changed at different times in the cored interval, so applying one rate to the whole core is also not a good idea.

Sediment accumulation rates in all dynamic basins varies even up to several magnitudes. A small pond with virtually no wind induced water motion may show rather uniform deposition but even then the supply of sediment to the pond may vary from year to year.

Micheal mentioned that two cores taken less than a kilometre apart showed differences in deposition rates. My own experience especially in the large deep basins of the Baltic Sea has clearly shown that no two cores are identical even though taken within a few hundred metres of each other. In many cases you can compare sea floor deposition to snow forming drifts on land.