Should a washer go on nut side, or the bolt side?
Washers are used for multiple purposes when mechanical parts are assembled using bolts and nuts. Here are some usages and as you can see it is not likely that a generalized answer can be devised to directly answer your question!


Some washers have a special design that attempts to help keep the nut and/or bolt from coming loose. Known as a lock washer these may be a split ring type, star type, wavy or any one of a number of other designs. A lock washer will be used with the part of the assembly that most likely could turn such as the nut. It could be used under the bolt head in instances where the bolt screws into threads in one part of the assembly.
A washer is sometimes used to protect the surface of the assembled parts. A nut or bolt head being turned during the tightening process can mar the part surface around the hole and a washer can be used to take the abuse as opposed to the part. This may be particularly applicable when the parts are a softer material such as plastic, brass or aluminum and a washer made of a harder material is used.
Sometimes a thin material is part of the assembly and a larger diameter washer is used to help distribute the pressure of the tight bolt across a larger part of the thin material. In these cases the washer would be used against the thin material. It can help to keep the material flatter or to prevent pullout of the fastener.
There are times when a large diameter washer used as in #3 above may be used with yet another washer smaller in diameter washer on top of the first. This would be done for cases where the larger diameter washer may tend to mushroom when the bolt is tightened.
Assemblies often have a slotted or oversize hole on one side. This is designed to allow for tolerance in the hole size and location in the parts being assembled. Another variation of this would be a long slotted hole that permits the adjustment of one part relative to the other before the bolt and nut are fully tightened. A washer would be used over the slotted hole to allow the pressure of the tightened bolt to be spread across the slot and up onto the part itself. In some instances the washer will also help to prevent the turning bolt head or nut from deforming the slotted part during the tightening process.
Some assemblies are designed to have the parts move with respect to one another when the bolt is actually tight. The travel of the nut may be restricted by a shouldered design of the bolt or a sleeve can be installed around the bolt in the hole. In these cases a washer is often used to keep the moving part from wearing the bolt head or nut. The washer may be made of a material designed to reduce the friction between itself and the moving part. This can also help to keep a turning part from trying to loosen the nut through friction.


Many times washers will play a role that is a combination of the above usages. One common example is to see a lock washer used along with a larger diameter flat washer.