Very basic level question

your track level SLIDERS have no affect on recording levels.  you set your recording levels on your input signal chain, i.e. your guitar pots, your preamp sliders and your soundcard input levels.

since you have a presonus firebox, you can record at 24bit, so if you're not doing that, then I encourage you to do so and if you're recording in 24 bit then I would set the input levels of the guitar tracks (and all audio input vocals, bass, whatever) to peak between -6dB and -12dB and you can go even lower if you want. 

that will give you plenty of headroom for mixing.

during mixing you should never have any track or bus exceed 0dB, but again, I'd advise no more than -6dB for each track/bus until you get to the master.  that gives you more head room at the master for summing.

if you do go over 0dB on tracks and buses it's not that big of a deal in sonar as long as you have the 64bit engine on because sonar will take care of the digital overs in math processing.  but I still think it's best to practice "safe bits" and keep them in the clear of 0dB anyway myself.