The OP was asking about hiss, not 'background noise'.
I'm assuming you are recording with room mics in order for these things to pose a problem. How about handheld, lavalier or headset mics? Again, the results will be much better if you fix it at the source.
Yes, and platitudes are by far the best tools for recording. Sorry, there are times when you have no control over the details, but good results are required. Noise reduction is useful, in combination with high-pass filters, notch filters and many other tools. "Problems should be fixed at the source" is a nice epigram to offer to people who have no experience, but I record on an actual planet. Those times when I have the necessary control (as in my studio) noise is not a problem because I know what steps to take when noise arises. And the action I take is NOT "fix the problem at the source." That's meaningless. What I do is hands-on tweaking of many different sorts to ensure that the area is noise-free, at least as far as it ever can be.
So the real problem with your advice is that it offers nothing. If you were to provide strategies to a beginner on how he can "fix it at the source" you would be doing him (or her, no sexism intended) a favor, larger or smaller depending upon your own expertise and his (or her) talent for implementing it. You haven't yet achieved that.
But the difficulty in assigning settings to unknown noise on an existing recording is that the range of possible problems is vast. Luckily, the software tools available are powerful and adaptive; even so, there are situations where you must trade that last increment of noise reduction for intelligibility or musicality.
Unless I have recorded in a well-treated room, I generally run each track through a high pass filter set just below the lowest "desired" frequency content in the track, to get rid of boominess from the room, which will make your recording sound tubby [and don't tell me, "always record in a properly treated room"].
Then, with the noise reduction program in Cool Edit Pro/Adobe Audition, always begin with a "quiet" segment where no transients were recorded; sample that, and then use the "%" setting to get rid of as much noise as possible while retaining as much tonality as you can.
This often requires a rather lower setting than you think, and can be influenced by the busy-ness of the arrangement and other factors.
I prefer to do all this before tweaking the EQ of a track, because some of the EQ will be undone by the NR if you're not careful.
Later versions of AA (1.5?) have a nifty lasso tool that allows you to find transients (audience coughs, for example) in spectral view, drawing around them, and clicking on "Fix one click" (or something similar -- I'm not at my recording computer); the software clones the audio around the noise and makes it "sort of" disappear. If it worked perfectly, you'd never hear a cough on a concert recording! But used sparingly, it will help.
Good luck. Do a little at a time and don't commit to the NR track until you have satisfied yourself that you have preserved everything important.
