Grouping:
I dont group a lot of sounds together. So although this is a post about grouping, its more about how I dont group things so much.

I will split this into 2 possible subjects:
1.Grouping stuff that then gets effected.
2. Grouping tracks or rather outputting tracks into a group so that I can deal with them all at once level wise.

1. Ok, you can do lots of crazy effects on things, like drums, but grouping and compressing drums I used to think was a good idea, but then if that includes lower sounds which have some function such as the kick or snare, then compressing that group could be or will in some way taking away the punch (the difference between loud and quiet). Great to group some lead sounds to keep them under control, or vocals wow, what a minefield (I need to do a seperate tutorial on vocals which indeed does include grouping- I have a lead group, backing group and harmony group if applicable all going into an overall vocal group to control stuff). Anyway, I feel I do less of this. You could have a group that is then sidechained (strings, effects etc).

2. Sending tracks to a group to control them on one fader is very handy. Great example is when you have 3 lead sounds (1 might be the main lead sound that you love, another might be something to help an area where that lead doesnt work so well, so if the main lead is really tearing, the support one might be warmer, but quiet in the mix and the third one might be really bright and sidechained off the kick and only comes in when the beat is playing, to give an effect of being loud but making more 'happen' when the beat is in). In this case, then I try and get the mix right, then send them all to a group. That way, in for instance a hard trance or hardcore or hard house track when it kicks in and you have a problem 'the leads dont stand out' or 'the leads are making the top end of the beat sound dull' you can just get on that group where you are happy with the mix in itself and brighten it up OR use a high shelf to knock the very top end off (whilst probably adding some 5kHz to give the impression of brightness but the top of the beats is not so disturbed).

So that's what I use groups for.