I am trying to get some information/guidance on the use of washers under nuts on flanges. I have reviewed a number of references, but none seem to provide definitive or prescriptive guidance.

ASME B16.5
Flanges to this standard installed in piping systems do not typically use washers. Washers are not discussed or included anywhere in the document.

ASME VIII Div I
UG-13 (b) The use of washers is optional. When used, they shall be of wrought material.

UCS-11 - this gives material requirements for nuts, bolts and washers of various types.

App 2 S2-2 Materials (e) Bolts, studs, nuts, and washers shall comply with the requirements in this Division. .....

Piping Handbook (7th Ed)
Chapter A7 Bolted Joints - this states that flat washers are used principally to minize embedment of the nut and to aid torquing. Plain washers are manufactured in accordance with standard ANSI/ASME B18.22.1. Hardened washers are utilized in high-torque applications.

Publication No. 009/98 Guidelines for Safe Seal Usage - Flanges and Gaskets (European Sealing Association)
4.5. Washers
Note that in the diagrams in the previous section, the fasteners (bolts or studs) are equipped with washers. These are important, not only to distribute the load more evenly, but more importantly to assist in the tightening process, by allowing more consistent torquing of the nut along the thread. Flat, hardened washers should always be used with fasteners, because they offer a number of important advantages. Namely, washers can:

• reduce significantly the friction between a turning nut and the joint components. This improves the consistency of the torquing operation, so improving accuracy and repeatability, while reducing the torque required
• reduce problems of fatigue by spreading the load placed by the fastener on the joint
  make the interface forces between joint components more uniform, which will improve gasket performance
• bridge slotted or oversize holes, facilitating assembly of poorly mated components
• prevent damage to joint surfaces
• reduce the amount of embedment amongst joint components, thus reducing relaxation after tightening

There are obvious places for using washers such as on GRP and plastic flanges. My question is more directed at metallic (B16.5) flanges.

Does anyone have any other useful references that may give guidance on this issue?

Read any of the books written by John Bickford "An introduction to the bolted joint".... he has various editions out they are all excellent!

See PCC-1. Washers should be through hardened. They are used to improve the torque to tension relationship when torqueing the bolt by having the nut turn against a hardening washer rather than the flange surface. Note that lubrication between the nut and surface it is turning against should be used in either case. Consider for critical joints or if there are issues with the flange surface.

Sorry Chuck I keep forgetting about PCC-1, read both!