Some fish will jump, some won't. Some types are more inclined to jump than others. Some people have more problems with "jumpers", while others rarely ever have that problem.
A glass top will help prevent evaporation and jumpers, but will cut down on the light getting into the tank (especially if not kept clean).
Many folks here go open-top, some don't.
There's a trade-off, so you'll need to figure out what you want to do......
