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It is up to you. For most of my brewing measurements, the 150 ml falls just about at the edge of my accuracy in volume measurement. As such for the few times I use dry yeast, I don't bother to add the 100 ml to 150 ml of water I use for rehydration. If you want to be absolutely accurate, you can add that as top off water to fermenter. Your choice, there is no wrong answer here.
If you look at it practically, 150 ml in a 10 liter batch (my standard batch) is only about 1.5% drop in starting gravity. This does not even change the measurement with my hydrometer. If I had one of the newer digital hydrometers, I would probably factor that drop in, as it would become measurable in my process.
You could also call it a yeast starter and check the "Add starter to bottling vol" box. Then BeerSmith knows that this added volume is not part of the batch volume, but is added to the fermenter after the batch volume has been measured. It really doesn't figure in any calculations then except for the Est Bottling Vol.