Should I tin these wires?

Tinning wires for these terminals has a couple of disadvantages:

• The contact will be more tangential to the cylindrical profile resulting in a smaller contact area than if the wires were untinned and allowed to squash into a more rectangular shape.
• If, for any reason, the joint gets hot the solder can start to soften, flow slightly and terminal pressure will decrease leading to further exacerbation of the problem.

On many industrial installations the wires will be pin-crimped before insertion. This reduces the risk of stray strands left out of the terminals. Some crimpers result in a square cross-section on the crimp and these work well with the flats grips on the terminals.

You should absolutely NOT tin the wires.

You should use a bootlace ferrule if you want to do better.

They will save you trouble when building, and ferrules make maintenance much easier. When you have to re-insert a wire, inside a machine where you can't see F.A. , you bless the guy that used ferrules.

On the top one spot, with gold star by the way, of the list of reason to NOT tin wires is fatigue.

When you tin wires, the tin creeps into the insulation, creating an extra-hard bit (solder + wire + insulation), which acts as a fulcrum onto the bit just past it (only solder + wire), which is weaker and now operates mechanically much more like a single conductor than strands. If you move or even microscopically vibrate two wires, one solid/single-strand and one multi-stranded, the single stranded will take much less time to break or deform. Deforming can loosen contact, and is much more likely with a soft metal like lead or copper than with steel blends often used for ferrules.

On the second spot is the point Transistor makes about contact profile, though solder is often soft enough to give into a nice flat profile that is better than crimped ends.

Another reason (I guesstimate about 4th or 5th on the list) to crimp specifically has to do with uncoated copper strands. If you touch those, they will over little time get black or green copper salts on their outer layer, neither of which groups is great at electrical conduction. Cutting off the strands and twisting them with your fingers = bad idea. This is much less of an issue if each strand is covered with tin in the factory, but still something to think of. If you strip the wire and put on a ferrule and crimp it, you can do that without touching the wires.