OK... bare with me here...
You could just route each or your 8 audio tracks straight to the main bus. Or you could use your aux busses to groups for sub mix or effects purposes... the opportunities are endless...
Is there a particular reason that you are using 4 aux busses? Do you really need them?
So when I mix, what channel am I supposed to look out for, the master channel in the submix or the main channel that tells me the output to my sound card?
I'm not sure what you mean when you say "main channel"...
If you are using the aux busses for a submix (groups) then you can disable the output to your master channel from each audio track because you are routing them to either one of the aux busses. The aux busses are then routed to the master channel. Actually, I'd need to check on this statement cos I haven't done it myself before and it sounds a bit daft turning off the output to the master bus.
Anyway, personally, I'm not sure why you need to do this. You are only using 8 audio tracks (presumably stereo which give you a total of 16?) so it's not that complicated. I would only bother with submixes if I was considering grouping say vocals and backing vocals on one group, guitars on another, drums (probably mic'd with 5 mic's) on another and keyboards on yet another. But you can also manage without grouping. All grouping does, as far as I can tell, is give you the option of setting relative levels of items in each group and then mixing all the groups together for the main mix. I'm in a four piece band with vocals, drums, guitar and key and as yet I haven't ventured into using groups.
OR
You could use the aux busses for various effects and you would use the aux send pots on each audio track to set the level for each effect.
Actually, I should perhaps duck out of here now cos I've just realised that there is still much for me to learn... it's quite a minefield ain't it? getting around all the bits and pieces of a mixer. I'm at work at the mo and I feel I can answer this kind of stuff much easier when I'm in front of my Sonar console.
As soon as I know more I will be back... ... ...