You can probably determine whether you will need “risers” (I call them “vents”) this way:
Place a small cylinder or cup (like a shot glass or battery) on the palm of your hand
Curl your fingers up, around the cup.
1. your palm represents the piece,
2. the cylinder is your glass sprue,
3. your fingers represent any part of the piece that, in the casting position, rises significantly above your palm.
If the fingers represent any part of your piece that can hold an air buble pushed into the mold in front of a mass of incoming glass, especially if those parts are small, you probably need a vent.
Here’s an example of where a vent (or four) might have helped: