Dark subtraction is mostly used for scattering in the blue band and ASTER does not have a band in this spectral region so it probably is not necessary. ASTER opens in floating point radiance so you can perform atmospheric correction from these bands. I would first Layer Stack the VNIR and SWIR bands if using something like FLAASH. Since ASTER 1B data is already georeferenced, you can't use RPC Orthorectification on 1B datasets like you can for 1A datasets. However, the Rigorous Orthorectification module says it supports ASTER 1B data in HDF format which means you would have to orthorectify the VNIR, SWIR and TIR bands separately. So for ASTER 1B data, I would first orthorectify it (if you plan on doing this) and then layer stack and atmospherically correct it. Mosaicking of multiple datasets would probably be last.