Hi Jim,
 
depends on what your assemblies look like. With the usual stuff (Rīs and Cīs and smaller (lighter parts)) on one side, usually called bottom side, itīs should really not be a problem at all, just do the heavy stuff last.
As mentioned before look after possible mechanical impact of the already populated side, especially during second reflow.
If you go for glueing and wavesolder SMD the same caution should be taken.
If the latter is true for you it might take a bit more effort to implement the process of glueing and handling your workpieces, with doubleside reflow itīs not that much of a problem ... except for your THT-parts.
 
Good luck
 
Wolfgang