Tips on setting levels in Cubase AI 6?

Well, I did some test recordings of me playing my drum set, and everything sounded fine. I have the gain knobs on each input on the UR824 at about half; there’s a light on each input that glows red if the input level is too high. I’m assuming this is the interface’s way of telling me that the signal is clipping at the ADC. In the program, I have the input level set at -6 db. Nothing sounds clipped. I just am wondering if there is anything else I should be taking into consideration (as far as setting levels) before I blaze ahead and start recording bass, guitar, keyboards, and vocals over the drums.

I’ve read some threads on this site and gearslutz.com where people have mentioned that I need to make sure that the signal doesn’t clip at the preamp level or at the analog-digital converter. The thing is, my interface has the preamps and the ADC all in one box. The only way I can affect that is with the afforementioned gain knobs on my UR824. I’ve heard some say that I need to make sure the signal peaks around -18dbFS, and I’ve heard others say that that is too quiet and I’ll need to later make the recording louder in the mastering phase, which may add unwanted hiss and noise to the recording.