Hi, although I have never use hfss or the other simulation software you mentioned but from my experience with winfeko, xfdtd and supernec, i believe if you don't specify any boundary conditions there is already default values built into the software this default will be applied to your simulation.
Theoretically, when using software to simulate radiation of wave through unbounded space you have to specify boundary conditons because of the limited resources of the computer you are using(e.g speed of the processor and RAM available). Also, if you run simulation without BCs, there will be reflection at the boundary plane of your simulation space because no specifying the BC will make the boundary plane behave like a PEC (perfect electric conductor) plane, and these reflections will corrupt the result you will obtain from the simulation. You can check Allan taflove book on FDTD: computational electromagnetics, chapter seven.
Hope I have been of help. Regards.