Forgot to ask what you show in! If I remember from previous posts, hunters/eq, right? On top of classics and medals I'm pretty sure you're supposed to wear them in Hunter Derbies. Not positive on that though (unfortunately there aren't any derbies at the shows in my area yet! :( )
It depends on what classes are available at the shows you're going to. There's a big show I'm going to in July that has a $500 Childrens Hunter Classic that I imagine you would do if you were going. The prize list says that proper attire is required (which according to USEF is just a hunt coat and breeches), but I know most people wear their shadbelles in that. There are several other Classic classes but I imagine you would only be doing the Childrens ones. Most of them start around 3" and go up to 4". The hunter derbies have 4" options so you probably wouldn't be entering in that until you were showing that height. I don't think you'd really need one until you start the big eq. But I don't know what the medal or zone finals in your area are like! They're pretty expensive for a nice one (and I think cheap ones would look, well, cheap!). I'd ask your trainer when you need to make that investment!
If you show on the curcuit (usually A or AA shows) you should wear one in classics and sometimes medal finals. Most people in my area, when they go to VA state finals, they wear them in the Stakes classes.