I encourage people to have a single point of contact (POC) with Shared Parameters. If a project needs a new parameter then see the POC for getting it added to the office Shared Parameters. Often this interaction makes a team aware of the fact that such a parameter exists or a similar one does for much the same purpose. Same goes for firms with more than one office. They just need to share a read only copy of the shared parameter file for each office so they can use them too. Call the POC the SP Czar or King or Gov... put one person who has the best handle on what exists and what's possible and what it's all about... Alfie.
If you have job or client specific sets of parameters, just create a separate group for them. Avoid having job specific things for generic things like length, width etc. Often this starts with documentation requirements, what has to appear in a schedule for example. If you and the office can define what information must appear in that kind of documentation you can map out a parameter strategy. Keep in mind that this situation is happening in every office and for every person that makes content... I think of it as the great big can o' worms that Revit and BIM have spun the lid off and hasn't resolved.