Here's an easy way to evaluate the action on your bass.
Play up and down each string. Do they buzz unacceptably anywhere?
No:
Are you happy with how the bass plays?
Yes: Congratulations! You're done.
No: Lower the string height a bit and repeat.
Yes: If you are getting buzz
On the low frets by the nut, but not on the upper frets by the body: Add some more relief to the neck, and repeat.
On the high frets by the body but not on the low frets by the nut: Take some relief out of the neck, raise the string height a bit, and repeat.
Everywhere: Raise the string height a bit, and repeat.
Only in one specific place on the neck: Sorry, but you have a high or low fret, and you will need some sort of fret leveling or re-seating---have a tech or luthier check it out.
When using this procedure, as you get closer, you can make smaller and smaller adjustments. Eventually you will get to the point where it's as good as it can be. If your neck is not warped or twisted and your frets are in good shape, even a cheapo bass can be made to play remarkably well.
In my experience, a bass set to Fender's set-up specs will only play cleanly if the frets are level, and on most regular-production basses the frets are a little uneven, so you get buzz when action is set at the Fender spec level (which is actually pretty low). I suspect most people could set up a bass to play pretty comfortably without getting it down to those specs. Of course, fret leveling is a good idea if you know your bass is a keeper, but you probably won't get that cost back out of it if you sell it.
