Unity Gain

My advice? Run the outputs of your synths at a reasonably healthy level. What's healthy? I'd say keep the volumes as they are on the original patch unless they are obviously distorting.
 
Next I would then adjust the levels coming into Sonar by using the trim controls at the top of each channel so that nothing peaks above about -12dB to -9dB. This assumes all of your track faders are set to 0.
 
Now you're in a good position to mix the project properly and having all your faders at 0dB as a starting point means you can fine tune the levels of individual tracks with the fader where they have their greatest resolution/sensitivity (due to the logarithmic nature of the Decibel scale)
 

Synths have a lot of cool things in them. Effects that change the output.

Syntorial is a cool program for learning how to get the synth sound you want.

Unless the output of the synth comes before a master limiter there is no difference between an output gain on the synth and the volume of the track. Lots of synths do have a final limiting stage.

If recording the synth's analog output, I prefer to leave the DAW's faders at unity and adjust the synthesizer's volume control to set recording level.
 
However, if the synth features s/pdif output, that's better because it's normally a straight digital-to-digital transfer, bypassing the DAC in your synth. The downside to s/pdif is twofold. First, you may not be able to set output levels at the instrument and might subsequently need to lower or boost the recorded audio in SONAR. Second, you have to switch your interface's clock source before recording via s/pdif; forgetting to do that, or forgetting to switch it back to "internal" will result in clicks in the audio.
 
Of course, all that's moot if we're talking classic analog synths. Some of the vintage machines have issues with hum, which might require setting their outputs higher than the normal line level. I'd recommend getting a DI box for recording analog synths.
 
As for mono versus stereo, it's generally a good idea to favor mono tracks. You'd intuitively assume that stereo tracks result in a better stereo mix, but it doesn't work that way. When you mix many stereo tracks, the end result is unsatisfying mush, panoramically-speaking.