MOTM

The 120 was one of the first kits I built and I did not solder the 
vias at all.  In theory since all the holes are plated through 
soldering should not be necessary.  However, it is always a good 
idea to solder them since the plating through process may be less 
than perfect and soldering insures a good connection between sides.  
Since those first few kits I have always solderd the vias with water-
clean solder as a last step prior to the final board wash.  If you 
decide to do this be sure you don't solder any of the remaining 
holes which require parts or wires to be added with no-clean.