[Healeys] shim value springs?

[john spaur]

I was looking for an oil pump and noticed Denis Welch had valve 
spring shims for sale. By coincidence I am rebuilding the engine head 
and noticed that it had valve spring shims which I must have 
reinstalled when I put NOS valve springs in the engine several years 
ago because the ones that were replaced were out of spec.

How does one go about determining whether shims are needed (or wanted 
for some reason)? How do you measure for them or what is the 
selection criteria?

Just curious.

John Spaur
'62 BT7