I do in some cases. I've actually converted a couple of old clients to Debian and showed them how to maintain it for what little they used their computers for, and effectively lost them as customers because they stopped having issues with their computers. I see that as a good thing though; I probably gained twice as many new clients on their word of mouth recommendations alone.

But it's not for everyone. Most of my clients in the past have had specific commercial software requirements, so I wouldn't even bring up switching platforms. These days it's easier, as everything is moving to software as a service and web apps, and they are already used to running their home office from their iPad or Android tablets. Still, I make sure that my end goal is to offer the best value to my client, not push potentially unfamiliar and net-cost-ineffective solutions based on software license alone.