Ethically.
The Medicaid question could in theory become a practical challenge. Need a lawyer to chime in on this.
So, let me just throw out a few thoughts. As an FYI I heard that the Agudah mentions what you are saying (your practical reason) as a reason not to go ahead and file. Though I don't really understand it.
Lets say "Yankel" makes 10k a year as a rabbi, additionally he owns a furniture store and makes 100k a year from that. Let's say he filed the 4361.
Now, on the 100k of self employed income he is paying the 15.3, and on the 10k paying nothing.
It's very clear that he gets the SS and other benefits from what he is paying/payed into the system.
So, on the income that he was allowed to opt out based on his own beliefs - he choose to do so. But he is not swearing that if needed he wouldn't take the benefits.