Not really taking money from the crew, just not giving the crew the same amount as someone who pays on board.
In general (not responding to you directly) We have PPD tips but will give additional in cash. Others will give more than us, others will give less. Either way does not bother me and is really none of my business. Everyone should give what they want to whomever they want and not be judged. If you want to give the cabin steward nothing, so be it. If you want to give them $20 pd, so be it. What someone tips or doesn't tip does not impact any other guest - so don't worry about it. As another said, you be you.