When making a polysulphide impression for a caste crown, custom trays are usually preferred over stock trays for all of the following reasons except
The use of a custom tray is recommended to reduce the quantity of material required to make impressions; thus, any dimensional changes attributed to the materials are minimized. A custom tray allows a uniform distribution of impression material between the tray and the object, which also improves accuracy. This is especially true for polysulfide impression  material. 
The use of custom  trays  for  polyether and addition silicone impressions is not critical, since these materials are stiffer and have less polymerization shrinkage than the polysulfide material. In addition, disposable stock trays work satisfactorily. Note that the use of less material in a custom tray reduces the compressibility of the impression, which can make removal of the impression tray more difficult. When severe undercuts are present, the use of a custom tray should be avoided. The stock tray should be rigid, thereby minimizing flexure of the tray during impression making.
Phillips 12 ed page no 156
Custom trays facilitate uniform contraction of impression material
Custom trays require less impression material
Stock trays may be short in the flange area
Impressions made in the custom trays are easier to remove from the mouth