Hard, medium and soft are all relative terms; and the precise definition of the melting point will vary between manufacturers. If in any doubt, it is very quick and cheap to compare the melting points of some small sample pieces of solder prior to attempting to solder a valuable piece of work.
It is generally accepted practice that the solder joint should be as reliable and strong as the design allows. However, if the article to be produced will be subject to periodic adjustment, it may in fact be better to create a slightly weaker joint. Softer grades of solder are generally weaker than hard grades of solder, it may be beneficial to use soft solder rather than hard as the joint will be easier to work on in the future.
