This question is one that many a site looking to implement a warehouse system forgets to ask!
It is a subtle question and can make the difference of a successful joined up system and two silos of information with users looking at two versions of the truth!
The answer to where should the stock information be held will be different depending on the size of the business, yet in most cases we would argue that the ERP/business wide system should own the stock. Be this a Sage 50 user or a Sage ERP X3 user, this must be the master record. The logic behind this is that for a business or the ERP system stock in the warehouse is only one facet of the business and whilst important to the warehouse user, someone in the accounts office would be more interested in the value of the stock as oppose to where it is, but both want to accurately know, how many are in the business.