Two Sided Jigger
You don't need to measure cocktails but you need to measure them to make them well. The 2 sided jigger is what the pros use these days, but has much room for error. However, the 2 ounce total angled jigger from OXO is great for home use because you can build the whole drink in the jigger.
Most cocktails are scaling down these days and that it a good thing. You don't need a 6 ounce martini on a bad day— you need 3 martinis. Smaller drinks stay cold and their quality will stay in tact while drinking. Thus, fitting your recipe into a 2 ounce jigger is recommended for people learning cocktail templates.
However, jiggers don't make drinks just "work" citrus changes geographically and even throughout he day, different brands make different cocktails and some recipes just aren't very good. Taste you drink to make sure they are good and measure them to know how to change them.