This is a question that seems to come up a lot among new sewers, and there isn't really a right or wrong answer. It honestly depends on what you're more comfortable with. Some people prefer to keep the spool of thread up so they can see how much is left and replenish it when needed, while others prefer to keep it down so it's out of the way and less likely to get tangled.
If you're still not sure which way to go, experiment a bit and see what works better for you. There's no harm in trying both ways and seeing which one you prefer. In the end, as long as your sewing machine is running smoothly and you're able to produce quality stitches, that's all that matters!