Coppering, by non electrical means, forms a very poor bond with iron or steel. The copper is deposited on the iron / steel due to the natural electro-potential difference between iron and copper. However, the bond forms quickly and is very poorly adherent. If the process is used to facilitate soft soldering, the result will not be as strong a bond as would result from properly cleaning and fluxing the steel and soft soldering directly to it. In the case of silver soldering, the copper does not improve the process or the bond. At the much higher temperature of silver soldering, the copper mearly alloys with the solder and results in a solder with a higher copper content. The bond is effected in the usual manner between the two items to be joined - that is by the solder flowing onto the surface of both parts. The copper plating bond is consumed in the process and no longer exists as a plated bond.
To prove how poor the bond is, clean and plate a nail with the copper sulfate solution and then try to scrape it off. It will peal off in sheets. Commercially, copper is plated from either acidic or alkaline baths. The acidic ones lay down a lot of copper fast, the alkaline ones are used to deposit copper of finer structure that adheres much more firmly to certain substrates, most usually iron or zinc based alloys. The acidic baths, if used to plate iron or steel directly, result in a plating that will flake off and fall to the bottom of the tank. To copper plate steel properly, an alkaline (i.e. cyanide) solution is used first, but requires the application of electrical current. After the initial layer is put down, the part may be plated further in either an alkaline or and acidic bath.
In Etruscan filigree gold work, copper salts were used with a hide glue to hold the extremely fine parts together. As the article was heated, the copper salt was converted to a copper oxide but the oxygen in the oxide was combined with carbon in the charring glue and went off as carbon dioxide. The residual copper alloyed with the surface of the gold to form a lower fusing alloy that effectively soldered the whole piece together without flooding the fine surface detail of the wires, grains, beads etc., into an unsightly mass.
Bottom line here, yes copper can be helpful in forming a high temperature brazed or silver soldered bond, but a quick dip in an acidic copper plating solution on steel will not improve the ease or strength of a soft soldered joint.
By the way, the Romans used the copper sulfate technique to plate iron ship fittings - sort of an ancient version of galvanizing. It did keep the iron from rusting quite as quickly as bare iron did. However, the coper was thin, did not adhere very well, and went away in fairly short order.
