You definitely don't want to throw the tea leaves into the water. It also depends on the base of the jasmine tea- most jasmine teas are green or oolong, I've never come across any other teas being infused with Jasmine (well maybe some herbals, maybe some straight jasmine flowers steeped). I would definitely use an infuser or even a paper filter to keep the tea leaves in. It's easier when it's all done to remove it.
As for temperature, you want to use the proper temperature depending on the tea- cooler 180 temps or so for green jasmine teas, you can go hotter with oolongs (depending on the color- darker- hotter, lighter- cooler like greens). Steeping wise, green jasmines are usually 3 minutes, oolongs 5 minutes. The key is the base tea.