Hello.
Yes homogeneous Neumann BC are automatically taken into account if nothing is
specified. This is related to the variational formulation of the finite element
method.
Nevertheless, be careful. In some situation the PDE formulation does not
ensure the use of the correct variational formula (this is the case for
instance of elasticity system). So, it can be better to use the variational
formulation which gives you more control on the way your problem is
discretized.
Best regards,
Stéphane.