I find that a feeler gauge is an essential tool for use with my shooting boards. While our shooting boards allow you to create precise 90 and 45 degeree angles, many times when you are fitting your stock into an opening, such as fitting a drawer or fitting a box top the opening you are fitting to is not precisely 90 or 45 degrees. To match the actual angle, I use a feeler gauge between my shooting board's fence and the stock.
PEC makes a great feeler gauge and here is why:
Made from the Finest Feeler Gage Stock Available, Hardened and Tempered