That's a hard question to answer, first, because it depends on what gear you have, second, because it depends on what vehicle you have it in, third, it depends on what kind of music you listen to, and finally because everyone's ears are different and what sounds good to, for example, me, won't to you. Not that any of us could tell over the internet in the first place.
If you've never done it before, or this unit at least is new to you then I strongly recommend you seek out a local car audio shop (try to avoid the big-box stores), pay them $30 to $50, and have them break out their test tone disks and multi-meters to tune it right for you. Most of them will let you watch and learn along the way since it's your ears they are tuning to, and doesn't take long. I did that with mine even though I've installed lots of car stereos, including my x920bt with an amp, sub, and new speakers. Sometimes it's just good to get another set of ears, especially ones that tune amps for a living, to listen in and help.
Now that I've done that I know what to do to tweak it further should the urge strike me.