The rule of thumb I have used to determine whether or not an activity needs its own fund is this: Are there (many) more than income account associated with this activity, and are there (many) more than one expense account associated with this activity.

Obvious candidates for having their own accounting funds are things like missions accounts, building funds, church schools, etc.

Some activities that may need their own funds are investment accounts, summer programs, or large book ministries.

The most important test if you are unsure is the number of income and expense accounts associated with the activity.