I usually go with around 0db, if you go much more you will usually clip. That applies to digital, but for analog you can probably go higher and get away with it.
To get the cleanest signal set all of your pre-amp levels to optimum, and then when the signal reaches your recorder you wont need to boost it as much. All amplifiers have a point where when they are driven they distort and produce excess electronic "garbage", one very rough rule of thumb is to go approx 5/8 to 3/4 of a pre-amps max. Most times anything above that produces that annoying hiss we hear in poor recordings, of course a lot of factors come into play here such as the class or type of amp and its power supply. Your ears are probably one of the best judges. You will actually hear the hiss increase when you reach that danger zone. I'm talking about listening to an amps output without program material.
If your material has big swings of volume, its probably best to get a compressor. Depending on how you adjust it, you can eliminate those peak zones where youre input would spike, and a compressor will bring up the sounds that are too loww as well.

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Smitty