Yeah, the "stop leak" stuff is a bunch of small particles that are supposed to clog and block any small holes to the outside world. There's no worry about blocking the thermostat because that's a *big* opening when it's open. Nor do you care about the thermostat not quite closing properly because it has a deliberate bypass passage so there's always some coolant flowing anyway.
On the other hand, solenoid valves sometimes have very small pilot valves (that switch water pressure that then opens the main valve). You wouldn't want to block that or the valve might not open. Nor would you want the main valve to leak when it's supposed to be off, since that might mean heat in a room that is already warm enough.
Dave