It's not just a matter of making the wires brittle (where the soldered together strands transition to the free ones, often under the insulation). The solder will easily cold-flow over time, removing a lot of the clamping screw pressure.
This is much more of an issue for higher current connections, but can still cause unstable connections at low current.
Crimped ferrules (on non-tinned wire ends) are an excellent way of terminating stranded wires for use in terminal strips.