Which scripts do I NEED?

[schuhplattler]

Some websites do me no good unless I allow certain scripts, and

Some websites (e. g. online newspapers, banks, brokers, and credit cards) include many many scripts.

Is there a quick way to determine which scripts I need?

If not, is there a guidebook that lists commonly used scripts and tells what each one does?

[barbaz]

Unfortunately, if you don't dig around in the page source code it's just trial-and-error.
viewtopic.php?p=73872#p73872 might help you reduce the amount of trial-and-error you have to do.