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This is subjective per say. But I am looking for other peoples experience.
If I am going to screw eight wires into daughter relay board's terminal block, is there a benefit or advantage to tinning the copper end before?
The environment that the units are to be installed is not near the coast and is not normally damp/wet. Rather dry and hot.
You MUST NOT fully tin the copper wires to be inserted into a screw down terminal block - that your days may be long on the face of the land.
It is permissible to tin the tip to maintain the wire shape.
The minimum possible amount of copper should be tinned.
Any competent regulatory authority will have this requirement as a rule in their system (see below)
The reason for the prohibition is that when you fully tin a multistrand wire fully, the solder wicks between the strands of copper and forms a solid block, part of whose volume is metallic solder. When you clamp the solder and copper bundle you tighten the screw or clamp against the solder block, and in time the solder metal "creeps" under the compressive forces and the join loses tension. The wire can then either pull out or cause a high resistance connection with heating.
This is a genuine real-world issue and is covered by genuine real-world regulations in many countries.
If you want to do your installation a favor, use wire ferrules:
Image source: http://de.wikipedia.org/wiki/Aderendh%C3%BClse?uselang=de
Do not, ever, tin the wires. A tinned wire will slowly give way to the screw's pressure and eventually become loose. The contact resistance will rise. In the worst case, your contact will become hot and cause fire.
If you don't have wire ferrules at hand, just twist the stranded wire before putting it into the terminal block. While not recommended because some strands might cause shorts to neighboring terminals, this will still be far better and reliable than a tinned wire.
If they're stranded wires: don't tin them! The tinned whole will be soft and the screw will become loose in no time. Instead crimp a ferrule on them.
Also for solid wire I don't think tinning is needed. Think about it: have you ever seen electrical wiring in a house where the 2.5mm\$^2\$ or 4mm\$^2\$ wires were tinned before fixing into the wiring cabinet? Yet these connections are used in all kinds of environments and for tens of years.
