Lately I have been applying different types of mix buss processing tools to my subgroups. One in particular are these API 550a EQs on my vocal buss. They sound good on anything but they sound so good on vocals. There are digital versions of this eq but in my opinion there is no comparison to the real thing. They are so smooth and rich that adds vibrance and character applying a overall cohesive tonal sound to the vocal mix.
For the majority of the time I have been a mixing engineer, I did not use subgroup processing. I mixed very straight forward on an analog console and at times created only back ground vocal mix buss sub groups when I was running out of console channels. I will also now apply other things like a buss compressor which is really helpful getting vocals to sit on top of a mix.