A V6 with 60 degrees between the banks (like the
2.8/3.1/3.4l) should be in good balance  (likewise 120
degrees, but this hasn't been seen since a Ferrari race
engine).  Those that have 90 degrees between banks usually
require some sort of balancing, either a balancer shaft
(4.3l) or offset crankpins (3.8l).  This is the historical
version--with the latest CAD, some of this can be overcome
by other design additions (like the VW VR6).  There's more
to say in a technical way about this--this is the "lay"
version.